Handle .asmdef files and player projects

Player projects add a second project with the same files as a "normal" project. If a `.asmdef` file is included twice, it is treated as though the assembly definition name maps to multiple files, even though the PSI source files point to the same file on the file system. This causes problems such as breaking the "convert to GUID reference" quick fix (see [RSPL-6988](https://youtrack.jetbrains.com/issue/RSPL-6988)), which has multiple GUIDs to choose (although they're all the same GUID).

Properly handle player projects, so that these "duplicates" don't cause problems. Check other areas for similar problems. E.g. does resolve correctly? Does it return multiple candidates, or map appropriately? If it returns multiple candidates, is this being shown correctly as an error?