Custom PSI module for external assembly definition files

[citizenmatt]

Resolve errors are currently disabled for .asmdef files (#701), as we can't resolve assembly definitions in .asmdef files external to the project, i.e. referenced packages and dependencies. To correctly resolve this, external .asmdef files need adding to a custom PSI module to make them available to the definition cache.

[citizenmatt]

I think it would probably be better to create a new PSI module for each assembly definition in a package. This would fix this problem, and would also allow us to specify defines for the files in that package, known references, and provide much better navigation, etc. See #710