Don't confirm pinning invariant packages

[dra27]

Related to #4569, but not itself a bug. Pinning an invariant package still displays this prompt:

$ opam pin add ocaml-base-compiler --dev-repo
[WARNING] Package ocaml-base-compiler is part of the base packages of this compiler.
Are you sure you want to override this and pin it anyway? [Y/n]

This prompt made sense in 2.0 where the base packages were locked, but it has no place any more with the glory of switch invariants!

Combined with #4569 and forthcoming amendments PRs to ocaml/ocaml and ocaml/opam-repository, this makes pinning the compiler work naturally! 🎉

[rjbou]

Should we ask were the pinned package & version are not consistent with the invariant ? We can't detect all underlying conflicts without launching the solver, but at least check with the invariant formula. Ftm it is pinned but uninstallable.

[dra27]

I think it's OK as it stands:

dra@thor:~/opam$ ./opam switch invariant
["ocaml" {= "4.12.0"}]

dra@thor:~/opam$ ./opam pin add ocaml-base-compiler --dev-repo
[ocaml-base-compiler.4.12.0] synchronised (git+https://github.com/dra27/ocaml.git#4.12)
ocaml-base-compiler is now pinned to git+https://github.com/dra27/ocaml.git#4.12 (version 4.12.0)

[ERROR] Package conflict!
  * No agreement on the version of ocaml:
    - (invariant) → ocaml = 4.12.0
    - ocaml-base-compiler >= 4.12.0 → ocaml = 4.12.1
    You can temporarily relax the switch invariant with `--update-invariant'

[NOTE] Pinning command successful, but your installed packages may be out of sync.

dra@thor:~/opam$ ./opam install ocaml-base-compiler --update-invariant

<><> Synchronising pinned packages ><><><><><><><><><><><><><><><><><><><><><><>
[ocaml-base-compiler.4.12.0] synchronised (no changes)

The following actions will be performed:
  ↻ recompile ocaml-base-compiler 4.12.0*
  ↻ recompile ocaml-config        2                [uses ocaml-base-compiler]
  ↗ upgrade   ocaml               4.12.0 to 4.12.1 [upstream or system changes]
===== ↻ 2   ↗ 1 =====
Do you want to continue? [Y/n] y

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
⬇ retrieved ocaml-base-compiler.4.12.0  (git+https://github.com/dra27/ocaml.git#4.12)
⊘ removed   ocaml.4.12.0
⊘ removed   ocaml-config.2
⊘ removed   ocaml-base-compiler.4.12.0
∗ installed ocaml-base-compiler.4.12.0
∗ installed ocaml-config.2
∗ installed ocaml.4.12.1
[NOTE] Switch invariant was updated to ["ocaml" {= "4.12.1"}]
       Use `opam switch set-invariant' to change it.
Done.

dra@thor:~/opam$ ./opam pin remove ocaml-base-compiler --update-invariant
Ok, ocaml-base-compiler is no longer pinned to git+https://github.com/dra27/ocaml.git#4.12 (version 4.12.0)
The following actions will be performed:
  ↻ recompile ocaml-base-compiler 4.12.0
  ↻ recompile ocaml-config        2                [uses ocaml-base-compiler]
  ↘ downgrade ocaml               4.12.1 to 4.12.0 [upstream or system changes]
===== ↻ 2   ↘ 1 =====
Do you want to continue? [Y/n] y

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
⬇ retrieved ocaml-base-compiler.4.12.0  (cached)
⊘ removed   ocaml.4.12.1
⊘ removed   ocaml-config.2
⊘ removed   ocaml-base-compiler.4.12.0
∗ installed ocaml-base-compiler.4.12.0
∗ installed ocaml-config.2
∗ installed ocaml.4.12.0
[NOTE] Switch invariant was updated to ["ocaml" {= "4.12.0"}]
       Use `opam switch set-invariant' to change it.
Done.

I think at each stage opam explains what you need to do and in future you can remember to have opam pin add ocaml-base-compiler --dev-repo --update-invariant?