Instead of setting the `MicrosoftWindowsSdkWin32MetadataBasePath` msbuild property, which is a scalar and only allows one assignment, we now add to an msbuild item list so that projections (e.g. CsWin32) may consume many such metadata winmds in one build.
This makes projections that depend on this package automatically get the effect of this package's build folder in receiving projects, making it more likely to work the first time they try.