New `alr check` command

[Fabien-Chouteau]

In the context of Alire integration in IDEs there is one feature that would help make a better experience for users, that is to be able to check the validity of manifest(s).

$ alr check --manifests

This would only check correctness of the manifest(s) without performing any other actions, without creating/modifying anything on the disk. We should also improve the error messages associated with bad manifests.

There are other checks that we could add to this command, in particular for the index validation checks.

• The detection of bad crate names and typosquatting:

  $ alr check --crate-name=Gtkaba
  ERROR: Uppercase not allowed in crate name.
  $ alr check --crate-name=gtkaba
  ERROR: Are you trying to fool gtkada users?
  

• The validation of maintainers for a new release or modification. (The current solution fails from time to time because the TOML parser is not handling our manifest correctly.)

  $ alr check --crate-maintainer-gh=Fabien-Chouteau
  ERROR: Fabien-Chouteau is not a maintainer of ...