Validation rule to prevent any version

[matthewnitschke-wk]

pubspec.yaml files technically support any as a possible version:

some_dep: any

some_dep2:

some_dep3:
  hosted:
    name: some_dep3
    url: https://some_url.org

While this is valid for development use cases, this will fail during publish to pub, and would be nice if dependency_validator could catch

Consider this as an opt-in functionality

[matthewnitschke-wk]

This work was spiked out in this closed pr: #97