Resolve builtin packages to local package cache

[citizenmatt]

It looks like Unity copies "modules"/builtin packages to the local package cache. Rider resolves these packages to the builtin packages folder in the application folder. It might be better to resolve to the package cache. If the packages are compiled by Unity, rather than using precompiled DLLs in the app distribution, then we should resolve the package manager to the files/folders used to generate the reference assemblies. This will reflect any changes made by the API updater (unlikely - they've been copied from the app distribution) and will match up files with PDBs, meaning we can set breakpoints. With quick testing, it looks like navigation works better with the cached files, too.

[citizenmatt]

More detail: builtin packages are copied to Library/PackageCache and compiled from here (with the result stored in Library/ScriptAssemblies). This happens in Unity 2020.3, and has been back ported to 2019.4. If we open files stored in the app distribution, breakpoints don't resolve because the PDBs point to the files in Library/PackageCache.