Offer to remove autopin pkgs on install failure

[NathanReb]

This PR adds a prompt when an opam install command fails to install packages or is aborted, offering them to remove packages that were automatically pinned as part of the process.

This mostly affects installation of local packages, i.e. when opam install is passed a file or directory as this triggers an automatic pin of the given opam file or any opam file found within the directory, alongside their pin-depends.

It allows users to quickly clean up if they realize that was not their intent or if they are unable to proceed with the install.

Queued on

• Remove unnecessary use of chdir #6910

[NathanReb]

I'm opening this as draft because I have some questions regarding what should be done in those cases:

1. when --check is passed and everything is installed already. I'd assume if this is the case, then nothing has been pinned but I might be wrong about that.
2. when --check is passed and some packages must be installed.
3. when dry-run or show
4. when simulate (i.e. --check, --deps-only or --depext-only

1 and 2 are kind of a specific case of 4. If my understanding of 4 is correct, no pin is actually written to disk in that case and therefore there's no clean up left to do.

I also need to correctly update the tests so that the installation aborted test makes sure everything is correctly cleaned up, e.g. by adding pin-depends into the mix. I'll of course have to test the scenarios described above as well.

[NathanReb]

I updated the code so it unpins only when the user aborts the install and does so without asking for confirmation. The test was updated accordingly and I added a pin-depend in the mix to make sure it was also unpinned correctly.

[kit-ty-kate]

Could you split the addition of the new test from the fix? Doing it this way makes it very easy to understand what the fix changed by simply reading the diff

[kit-ty-kate]

While the PR looks reasonable overall i'd like a bit more time than we have to review it and think it over, so i'm bumping this to the 2.7 milestone