Remove dead code from PackageSpecReferenceDependencyProvider, and use PackageSpec's TargetFrameworkInformation in GetReferenceNearestTargetFrameworkTask

[nkolev92]

Bug

Progress: NuGet/Home#5154

Description

These changes are part of #6972, which has lots of other changes, so I'm trying to minimize risk by splitting refactoring and implementation.

In PackageSpecReferenceDependencyProvider

• Clean-up getting dependencies from external ref. This used to be a project.json thing where xproj deps came through msbuild, but we no longer have that.
  This allows us to remove the GetSpecDependencies calls from this codepath which becomes relevant in the next PR.
• We also add the DotnetCLITool into the way we fetch dependencies for PR. DotnetCLITool restore uses a created package spec, so as long as we create it and restore correctly, it's all fine. This doesn't touch that.

In GetReferenceNearestTargetFrameworkTask

• Use proper TargetFrameworkInformation and PackageSpec to avoid calling GetNearest in a custom way.
  In Full restore support for aliasing - make the assets file aliasing aware #6972, matching by alias becomes a thing, which means we need a TargetFrameworkInformation based call to avoid code duplication and potential bugs.

Once this is approved, I'll mark #6972 as ready for review.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.

[nkolev92]

There's a secondary issue here.