.cmt files from install directories

[mshinwell]

At link time record the paths used to locate various compilation artifacts; these could then be used to augment the .cmt search path in the debugger. This should mean that OPAM-installed packages with .cmt files in their install dir should be able to be compiled without OPAMKEEPBUILDDIR=true and yet still be debuggable.

OTOH, source files are needed too for debugging. Is it reasonable to expect those to be present in the install dir?

[dbuenzli]

I implemented support for installing .c, .ml, .cmt and .cmi files for any topkg package that use the high-level ocaml and C library descriptions. See this comment for more details.

You can enable this in a switch by setting the environment variable TOPKG_CONF_DEBUGGER_SUPPORT to true in the switch with topkg's next release.