In order to avoid batching the
_GetProjectReferenceTargetFrameworkProperties target for each reference,
the ProjectReference protocol can be amended to return an item from
GetTargetFrameworkProperties instead of a semicolon-delimited list of
key-value pairs. This allows a single build request to be sent to the
engine, and allows resolving references in parallel on multiprocess
builds.

The previous commit is a breaking change to the ProjectReference
protocol, requiring a .NET Core SDK change to return the now-expected
structured data rather than the old semicolon-delimited string.

That means, for example, that MSBuild v15.5 couldn't build a solution
containing a full framework project that referenced a .NET Core SDK
2.0.0 project.

To avoid this, reconstruct the new structured data from the old return
value:
* Allow the MSBuild engine to split the returned value on `;` and return
  multiple values.
* Batch over metadata common to those values to reconstruct a single
  item with the complete string return value.
* Parse that string into structured metadata on a single item, as though
  the project had returned a single item with metadata.
* Remove the now-redundant individual-valued items.
* Continue as before with adjusting the reference items based on the
  metadata.

In preparation for describing the changes to return values I'd like to make, I need to document what the old ones were.

Document the more-succinct-but-compatibility-breaking single-item-with-metadata return possibility for GetTargetFrameworkProperties.

* Call GetTargetFrameworks target to identify target frameworks in
referenced projects. Since TargetFrameworks is not set, this will avoid
a 2nd evaluation when the project single targets.

* Adds dependency on NuGet AssignReferenceProperties task.

Handle IsRidAgnostic

Fix MSBuildProjectName

See https://github.com/dotnet/sdk/pull/418/files

Doesn't look like the package is signed so this likely won't work.

This is needed to get the import to get the automatic import of
NuGet.targets.

Projects that opt-out of GetTargetFrameworks must be preserved (e.g. C# -> C++ references).