Warn for packages that are not pinned to a specific version

[drewbanin]

Feature

Feature description

When we make breaking changes to repos like dbt-utils, we invariably break someone's project. This happens when people 1) don't specify a version/revision for their package or 2) specify something overly broad, like master.

While this is pretty well documented (we could do a better job of documenting this in the actual packages), dbt should also WARN if a package is specified without a version/revision, or with the revision master. We should additionally add a config (project-level? profile-level? in the packages.yml file?) which opts out of this warning.

Who will this benefit?

Folks that are using packages and don't want external code changes to brick their projects.