In the FilterStaticFrameworks task:
* Convert Windows-style paths to Mac-style paths.
* Give a better error if a framework can't be found.
* Don't try to copy frameworks that don't exist on Windows to the Mac.
In the ExtractBindingLibrariesStep:
* Return a relative path to frameworks we've extracted to make things easier for
remote builds.
* In the _ComputeFrameworkFilesToPublish target, don't compute the source
directory for frameworks using RootDir + Directory, because some frameworks
may only exist on the mac, and RootDir + Directory will be a Windows path
when building remotely. Instead use 'Identity', which is a relative path and
will work on both Windows and Mac.
Fixes https://github.com/xamarin/xamarin-macios/issues/15289.
abbd6ef466