[gircompiler] Clean up parsing

[gircompiler] Clean up parsing

We never actually include multiple modules in the compiler,
so just nuke that.  Also rather than passing around GIrModule
consistently pass around a GIrTypelibBuild structure which
has various things.

This lets us maintain a stack there which we can walk for
better error messages.

Also, fix up the node lookup in giroffsets.c; previously
it didn't really handle includes correctly.  We really need to
switch to always using Foo.Bar (i.e. GIName) names internally...