Add in an API marker for API type packages.
This does several things:
1. Packages marked as API will generate a new "modid" (the provides) that can be depended on.
2. Packages marked as API will be searched systemwide, and anything declaring that package (even without the API marker)
will get an implicit dependency on the API package.
3. The API package itself will get a soft dependency on the "owner" package.

@Optional elements can refer to the API packages instead of a concrete mod as well.