Simplify method arguments for ComponentVersions class
Closes https://gitlab.com/gitlab-org/release-tools/issues/394
Previously, this class was working with either GitLab FOSS or GitLab EE.
After the single codebase, it only ever works with EE, so passing around
a project argument was unnecessary.
This also moves GitlabClient.file_contents to a non-delegate method so
that we can stop manually calling (and stubbing) project_path
everywhere we use it.