Fix bug in removal action

/path/to/xxx/x is installed to dst/x by default, so just remove dst/x instead of dst/xxx/x
Handle files/ sub-dirs for file overlays
Fix removal of pinned packages
Re-indentation (no semantic changes)
More consistent env variable scheme

OPAMDEBUG=1 was used to be the same thing --verbose
and OPAMDEBUG=2 to be --debug

now OPAMDEBUG=i is *always* --debug

use OPAMVERBOSE=i to se --verbose
Add debug message when trying to install an optional & non-existing file
Fix the files/ overlay creation to support sub-dirs
Fail it a compiler description file that we expect to read does not exist
Read all the metadata by first looking at the pinned overlay, then the overlay and then the global state
Clean-up in the handling of the overlay files copying