Implement parsing, AST, (de-)serialization, and placeholder global
metadata for linking against the libraries/frameworks for imported
modules.

The module map language is extended with a new "link" directive that
specifies what library or framework to link against when a module is
imported, e.g.,

  link "clangAST"

or

  link framework "MyFramework"

Importing the corresponding module (or any of its submodules) will
eventually link against the named library/framework.

For now, I've added some placeholder global metadata that encodes the
imported libraries/frameworks, so that we can test that this
information gets through to the IR. The format of the data is still
under discussion.