sagemathgh-37737: Follow up sagemath#36741: improve code structure
    
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From sagemath#36741 (comment)

> Overall, I would suggest to remove the direct and unconditional use of
multiprocessing from sage.features.
> Perhaps sage.doctest can put such Value attributes into features that
are to be hidden.

This PR implements the suggestion made in
sagemath#36741 (comment)

> I looked a little bit into it and here's an idea:
>
> * In features, implement a simple hide() / unhide() / is_hidden()
interface.
> * The only state is _hidden, not shared (for parallel doctesting this
will be set before fork so it's ok)
> * Implement AvailableSoftware.hidden() similar to
AvailableSoftware.seen() so the logic stays in that class and internals
don't leak to sage.doctest.control (if necessary use _seen[idx] to
record hidden state and/or number of hidings, or add another shared
array)

Fixes sagemath#37905


### 📝 Checklist

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- [ ] The title is concise and informative.
- [ ] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#37737
Reported by: Sebastian Oehms
Reviewer(s): Matthias Köppe, Sebastian Oehms

sagemathgh-37737: Follow up sagemath#36741: improve code structure
    
<!-- ^ Please provide a concise and informative title. -->
<!-- ^ Don't put issue numbers in the title, do this in the PR
description below. -->
<!-- ^ For example, instead of "Fixes sagemath#12345" use "Introduce new method
to calculate 1 + 2". -->
<!-- v Describe your changes below in detail. -->
<!-- v Why is this change required? What problem does it solve? -->
<!-- v If this PR resolves an open issue, please link to it here. For
example, "Fixes sagemath#12345". -->

From sagemath#36741 (comment)

> Overall, I would suggest to remove the direct and unconditional use of
multiprocessing from sage.features.
> Perhaps sage.doctest can put such Value attributes into features that
are to be hidden.

This PR implements the suggestion made in
sagemath#36741 (comment)

> I looked a little bit into it and here's an idea:
>
> * In features, implement a simple hide() / unhide() / is_hidden()
interface.
> * The only state is _hidden, not shared (for parallel doctesting this
will be set before fork so it's ok)
> * Implement AvailableSoftware.hidden() similar to
AvailableSoftware.seen() so the logic stays in that class and internals
don't leak to sage.doctest.control (if necessary use _seen[idx] to
record hidden state and/or number of hidings, or add another shared
array)

Fixes sagemath#37905


### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [ ] The title is concise and informative.
- [ ] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#37737
Reported by: Sebastian Oehms
Reviewer(s): Matthias Köppe, Sebastian Oehms

sagemathgh-37857: Replace special handling of optional extensions (bliss, coxeter3, ....)
    
<!-- ^ Please provide a concise and informative title. -->
<!-- ^ Don't put issue numbers in the title, do this in the PR
description below. -->
<!-- ^ For example, instead of "Fixes sagemath#12345" use "Introduce new method
to calculate 1 + 2". -->
<!-- v Describe your changes below in detail. -->
<!-- v Why is this change required? What problem does it solve? -->
<!-- v If this PR resolves an open issue, please link to it here. For
example, "Fixes sagemath#12345". -->

We relieve the distribution **sagemath-standard** (in both versions -
`SAGE_ROOT/pkgs/sagemath-standard` and `SAGE_ROOT/src`) from the duty to
build "optional extensions" based on what Sage packages are installed.

The installation is now done uniformly using the modularized
distributions **sagemath-bliss**, **sagemath-coxeter3** etc.

We introduce the missing features `coxeter3` and `sirocco` so that the
doctester does not have to rely on the sage-the-distro installation
records any more.

The wheels of the distributions now build correctly even when not going
through building an sdist first, which previously was required to apply
MANIFEST-based filtering. This is achieved using the new
`sage_setup.command.build_py`.

User-visible change:
- To install these options, use `./configure --enable-sagemath_bliss`
before building, or use `./sage -i sagemath_bliss` or `make
sagemath_bliss`.

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->

- Depends on sagemath#37737 (merged here)
- Depends on sagemath#37973 (merged here)
    
URL: sagemath#37857
Reported by: Matthias Köppe
Reviewer(s): François Bissey

sagemathgh-37857: Replace special handling of optional extensions (bliss, coxeter3, ....)
    
<!-- ^ Please provide a concise and informative title. -->
<!-- ^ Don't put issue numbers in the title, do this in the PR
description below. -->
<!-- ^ For example, instead of "Fixes sagemath#12345" use "Introduce new method
to calculate 1 + 2". -->
<!-- v Describe your changes below in detail. -->
<!-- v Why is this change required? What problem does it solve? -->
<!-- v If this PR resolves an open issue, please link to it here. For
example, "Fixes sagemath#12345". -->

We relieve the distribution **sagemath-standard** (in both versions -
`SAGE_ROOT/pkgs/sagemath-standard` and `SAGE_ROOT/src`) from the duty to
build "optional extensions" based on what Sage packages are installed.

The installation is now done uniformly using the modularized
distributions **sagemath-bliss**, **sagemath-coxeter3** etc.

We introduce the missing features `coxeter3` and `sirocco` so that the
doctester does not have to rely on the sage-the-distro installation
records any more.

The wheels of the distributions now build correctly even when not going
through building an sdist first, which previously was required to apply
MANIFEST-based filtering. This is achieved using the new
`sage_setup.command.build_py`.

User-visible change:
- To install these options, use `./configure --enable-sagemath_bliss`
before building, or use `./sage -i sagemath_bliss` or `make
sagemath_bliss`.

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->

- Depends on sagemath#37737 (merged here)
- Depends on sagemath#37973 (merged here)
    
URL: sagemath#37857
Reported by: Matthias Köppe
Reviewer(s): François Bissey