Fixes spack#2306

Any dependency explicitly mentioned in a spec string ended up with the
build and link deptypes unconditionally. This fixes dependency
resolution to ensure that packages which are mentioned in the spec
string have their deptypes determined by the dependency information
in the package.py files. For example if a package has cmake as a build
dependency, and cmake is mentioned as a dependency in the spec string
for the package, then it ends up with just the build deptype.

- Fixes issues with hashing where deptypes from one dependency would be
  overwritten by those of another dependency on the same package.

- Copying specs (`Spec.copy()`, `Spec._dup()`) did not handle deptypes
  correctly:

  - Copying "flattened" dependencies, but in doing so it collapsed
    multiple dependency relationships into one, losing some edge
    information in the DAG.

  - This gets rid of the `flat_dependencies_with_deptypes()` method,
    which doesn't really make sense, as it collapses edges. It's now
    reverted the original `flat_dependnecies()`.

  - `traverse_with_deptypes()` is now called `traverse_edges()`, which is
    when edge information must be preserved.

- This gets rid of the notion of "default deptypes" introduced in #2307.

  - Initially created Specs now have empty deptypes instead of "defaults".

  - Proper deptypes are added during normalization/concretization like
    everything else in the Spec class.  Empty deptypes mean deptypes are
    not yet specified, and the spec is abstract.

- Updated tests.

- Fixes issues with hashing where deptypes from one dependency would be
  overwritten by those of another dependency on the same package.

- Copying specs (`Spec.copy()`, `Spec._dup()`) did not handle deptypes
  correctly:

  - Copying "flattened" dependencies, but in doing so it collapsed
    multiple dependency relationships into one, losing some edge
    information in the DAG.

  - This gets rid of the `flat_dependencies_with_deptypes()` method,
    which doesn't really make sense, as it collapses edges. It's now
    reverted the original `flat_dependnecies()`.

  - `traverse_with_deptypes()` is now called `traverse_edges()`, which is
    when edge information must be preserved.

- This gets rid of the notion of "default deptypes" introduced in #2307.

  - Initially created Specs now have empty deptypes instead of "defaults".

  - Proper deptypes are added during normalization/concretization like
    everything else in the Spec class.  Empty deptypes mean deptypes are
    not yet specified, and the spec is abstract.

- Updated tests.

- Fixes issues with hashing where deptypes from one dependency would be
  overwritten by those of another dependency on the same package.

- Copying specs (`Spec.copy()`, `Spec._dup()`) did not handle deptypes
  correctly:

  - Copying "flattened" dependencies, but in doing so it collapsed
    multiple dependency relationships into one, losing some edge
    information in the DAG.

  - This gets rid of the `flat_dependencies_with_deptypes()` method,
    which doesn't really make sense, as it collapses edges. It's now
    reverted the original `flat_dependnecies()`.

  - `traverse_with_deptypes()` is now called `traverse_edges()`, which is
    when edge information must be preserved.

- This gets rid of the notion of "default deptypes" introduced in #2307.

  - Initially created Specs now have empty deptypes instead of "defaults".

  - Proper deptypes are added during normalization/concretization like
    everything else in the Spec class.  Empty deptypes mean deptypes are
    not yet specified, and the spec is abstract.

- Updated tests.