Allow MatchSpec in run_exports.from_package

[wolfv]

Currently we don't allow matchspecs in the run_exports.from_package list. We only allow literal PackageName.

Sometimes when templating it's easier to just use a matchspec, for example:

requirements:
  ignore_run_exports:
    from_package:
       - ${{ compiler('cxx') }}

To support this, we should consider to also allow MatchSpecs in this section.