Consider adding back ability to import other clowder.yaml files

[JrGoodle]

If this is added back, implement similarly to repo. Each file must be valid as a standalone, for easier validation. Add ability to remove projects from previous imports.

See: https://gerrit.googlesource.com/git-repo/+/HEAD/docs/manifest-format.md#Element-include

[JrGoodle]

see also:

• https://docs.gitlab.com/ee/ci/yaml/includes.html
• https://docs.gitlab.com/ee/ci/yaml/#extends
• https://docs.gitlab.com/ee/ci/yaml/#rules