check mode

[mmhat]

This PR adds a check command to the get-tested executable:
For instance, get-tested check package.cabal 9.12.1 9.10.1 will check if the tested-with stanza in the package.cabal file contains both GHC ==9.12.1 and GHC ==9.10.1.
To make the work with multiple package (e.g. in mono-repos) easier we provide an --from option:
get-tested check --from library.cabal executable.cabal will check if all compilers listed in the tested-with stanza of the executable.package appear in the one of the library.cabal. The intended use case is a CI pipeline, where the test matrix is generated from the compilers of the library.cabal, and we want to check that the executable.cabal package living in the same repository does not "lie" about the versions it was actually tested with.

Fixes #49