Make uninstallations remove changed files (and save them somewhere)

[kit-ty-kate]

If for whatever reason files in a switch were changed (be it by a manual install or other means), the files are currently left untouched and opam warns the user that some files weren’t removed.

In some cases this can make ocamlfind install fail (by default it fails if the files it tries to install is already present) and the reason why would be rather cryptic to users.

To fix this, opam could just default to move the changed files to a new directory (e.g. <switch>/.opam-switch/save) and tell the user about it at the end of the (un)install process. We might want to to an exception for files in <switch>/etc however, but I’m not sure.

Related to #4891
Would partially fix #3997