This makes the `PackFolder=buildTransitive` behave as if it were `build`, which makes the most sense.

Partially fixes #139
Fixes #179

Whenever we had `PrivateAssets=all` on a project reference (whether implicitly or explicitly), we used to unconditionally pass `PackFolder=$(PackFolder)` when resolving referenced content. This is correct whenever the calling project does indeed have a `PackFolder`, but not for packaging projects, since in that case, the packaging project itself doesn't have a value for that property (since it can pack any arbitrary content and has no primary output).

So we switch to the mechanism used already by the .NET SDK (and common targets) of using a metadata item with the full property assignment syntax. This means that if the metadata isn't present (which we now condition to the presence of a value for `PackFolder`), when calling `GetPackageContents` on the project reference, no value will be passed at all for that property, meaning whichever value is there already, will be propagated to the referencing project, which is exactly what we want in a packaging project scenario.

Fixes #139

We refine the support for packaging projects in this commit, by defaulting its PackFolder to Ignore, which becomes somewhat similar to None, but allows for differenciation.

Moreover, we test specific scenarios involving a packaging project becoming framework-specific (which we previously didn't explicitly support) by means of setting `BuildOutputFrameworkSpecific=true` at the project level. This requires re-targeting referenced assets whenever that property is true, in addition to non-packaging projects in that same scenario.

Fixes #182

Make it so that the default behavior for None items when the PackFolder for the project is build or buildTransitive the None matches the project. This is unless the item doesn't specify a different FrameworkSpecific or PackFolder metadata.

This fixes the default items that were not being properly populated for packaging projects too, due to the lack of a default file extension (meaning all default items end up being ignored).

Fixes #184.

A readme.md file at the project level will typically be automatically included as a None item,
which by default won't pack either.

This improves the inference by adding a `PackReadme=true` (by default) which controls whether a readme file (either explicitly specified with `PackageReadmeFile` as per the SDK Pack spec, or implicitly found alongside the project) is automatically packed and set as the Readme package metadata (as long as the project is being packed (IsPackable=true)).

NOTE: this is implemented in the Inference targets, so that when inference is turned off entirely,
this doesn't happen automatically, for consistency with other items inferences.

Fixes #185

This brings nugetizer pretty much to parity with SDK Pack for CLI tools too :).

We leverage the SDK tool packing targets to determine the files to pack, but otherwise preserve nugetizer's inference and packing behavior. Best of both worlds!

The new `InferToolContents` target is the one that transforms the `PackTool` target's `TfmSpecificPackageFile` into our own `None` items for packing. It can also be extended to trim the tool's contents via a custom target that removes items, such as::

```xml
  <Target Name="TrimRuntimes" AfterTargets="InferToolContents">
    <ItemGroup>
      <None Remove="@(None)" Condition="$([MSBuild]::ValueOrDefault('%(None.PackagePath)', '').Contains('/runtimes/'))" />
    </ItemGroup>
  </Target>
```

(this target would remove all runtimes from the tool package, but otherwise preserve the package contents).

Fixes #134