PKG foo should allow locate when source is available

[gsg]

At the moment, jumping to the definition of a package-provided identifier involves tracking down the location of the build directory and including it in .merlin by hand. It would be a nice if this job was handled by software.

[trefis]

This is now working on master.
However, I didn't do some clever hacks to add the build dirs of opam to the path (which was the "solution" on #234 ), I just expect the needed files to be distributed alongside the cmi/x/...
Indeed packagers can instruct opam to keep/distribute the cmt[i] and source files (look at the packages of Danial Bunzli), if they don't locate won't work.