behaviour when moving local switches around

[AltGr]

Issues arise when one has a local switch and e.g. renames its parent directory, e.g. foo/_opam becomes bar/_opam. It's difficult to handle, and in any case the OCaml ecosystemdoesn't perfectly handle relocation just yet, but we can probably do better (esp. when the shell hook in is place).

• _opam/.opam-switch/environment becomes wrong which leads to opam env malfunctionning. For this we should add a check that OPAM_SWITCH_PREFIX matches. First opam reads $OPAM_SWITCH_PREFIX/.opam-switch/environment and reverts it (here it can't be found) ; then it reads <current-switch-prefix>/.opam-switch/environment and applies it (here it's found but obsolete). This leads to opam env infinitely adding to e.g. PATH... Fixing OPAM_SWITCH_PREFIX would avoid that, but still not load a correct environment: the file should actually be re-generated, but that's not allowed from the shell hook (read-only mode), so maybe just bail out in that case ?

• the switch has registered in ~/.opam/config won't be correct anymore, but that should already be handled fine by the next opam calls (removed and added switches (once used) are detected and automatically updated)

Thanks to @samoht for reporting the original issue to me