Make ocamldebug -I auto-detection work with separate output directory

Summary: r14533 added the ability to ocamldebug to automatically detect
source files by recording their absolute paths and re-inserting those
as if they were -I directives at runtime. However, ocamldebug sometimes
needs access to build artifacts too, and if those artifacts are in a
different directory than source files, this heuristic isn't smart enough
to find them. But it's easy enough to use the same mechanism to also
record the build artifact locations too; we can use the same map, so
this doesn't require any format version bumps or anything.

Test Plan: Also included a new test case for all this -I logic.