Bug 567906 - Put pkg-config dependencies in .gir files

Bug 567906 - Put pkg-config dependencies in .gir files

When generating a .gir file, we now first parse all of our .gir includes
to pick up their <package> headers.  Then, we merge that with the set of
--pkg arguments passed to us, run pkg-config to gather the arguments,
and finally save the merged pkg-config list to our new .gir file.

This is useful for software which needs to map from .gir to pkg-config
in a programmatic way.