Pack import fixes

[lthls]

All opam packages using packs have been broken for a while (some of them since #871 has been merged). With this PR, we can finally compile most of the previously failing packages.
I think the code is still wrong in a few corner cases; I've left a CR though, and if someone knows how to fix them properly we should do it.

[mshinwell]

@lukemaurer could you please look at this?

[lthls]

It would be nice if we could get this merged before the week-end, so that we have updated opam health checks to look at on Monday.

[lukemaurer]

I think I made a mistake when I added the ~cmx_name parameter. If the import info we want is exactly what is in the ui, then we don't actually want to take comp_unit and cmx_name as separate parameters to get_unit_info - it should just take comp_unit and then let cmx_name = Compilation_unit.name comp_unit. Then we don't need either change here to get_unit_info - we want to check that ui.ui_unit is exactly comp_unit and the import we want to add (with or without .cmx) is comp_unit.

I think the solution is to have CU.which_cmx_file return the full CU.t rather than just the name. I'll make a PR.

[lukemaurer]

#1069 should have this covered.

[mshinwell]

Superceded by #1069 which is now merged.