Why:
* To implement robust cross-project go-to-definition in LSP
i.e you can jump to definition of any global name coming
from library dependencies, as well as from the local project files.
What it does:
* Modify `FC`s to carry `ModuleIdent` for .idr sources,
file name for .ipkg sources or nothing for interactive runs.
* Add `--install-with-src` to install the source code alongside
the ttc binaries. The source is installed into the same directory
as the corresponding ttc file. Installed sources are made read-only.
* As we install the sources pinned to the related ttc files we gain
the versioning of sources for free.
