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.
As it was, it could break if the argument was repeated more than twice.
When checking dot patterns, we need to check that no further holes are
solved, and that the pattern variable doesn't unify with some other
pattern variable, but if it had already made progress (either for a good
or bad reason) we missed this. Fixes#536
