Add stopgap option -fmodule-implementation-of <name>

This flag specifies that we are building an implementation file of the
module <name>, preventing importing <name> as a module. This does not
consider this to be the 'current module' for the purposes of doing
modular checks like decluse or non-modular-include warnings, unlike
-fmodule-name.

This is needed as a stopgap until:
1) we can resolve relative includes to a VFS-mapped module (or can
   safely import a header textually and as part of a module)

and ideally
2) we can safely do incremental rebuilding when implementation files
   import submodules.