Warn if an installation changes a package to avoid-version

[dra27]

Second item from Nov 27 dev meeting

"Post-process solutions to see if packages have changed to be hidden and warn (with a default of not installing)"

Writing this while 4.12 is still unreleased:

$ opam switch invariant
["ocaml"]
$ opam list
# Packages matching: installed
# Name              # Installed # Synopsis
base-bigarray       base
base-threads        base
base-unix           base
ocaml               4.11.1      The OCaml compiler (virtual package)
ocaml-base-compiler 4.11.1      Official release 4.11.1
ocaml-config        1           OCaml Switch Configuration

Present behaviour:

opam install ocaml.4.12.0
The following actions will be performed:
  ∗ install ocaml-options-vanilla 1
  ↗ upgrade ocaml-base-compiler   4.11.1 to 4.12.0~beta2 [required by ocaml]
  ↗ upgrade ocaml-config          1 to 2                 [required by ocaml]
  ↗ upgrade ocaml                 4.11.1 to 4.12.0
===== ∗ 1   ↗ 3 =====
Do you want to continue? [Y/n]

Desired behaviour:

opam install ocaml.4.12.0
The following actions will be performed:
  ∗ install ocaml-options-vanilla 1
  ↗ upgrade ocaml-base-compiler   4.11.1 to 4.12.0~beta2 [required by ocaml]
  ↗ upgrade ocaml-config          1 to 2                 [required by ocaml]
  ↗ upgrade ocaml                 4.11.1 to 4.12.0
===== ∗ 1   ↗ 3 =====
[WARNING] Your request requires installation of pre-release packages (marked "avoid-version")
Do you want to continue? [y/N]

i.e. with an extra warning and a change of default