Incomplete sources with remote execution

Background

Follow up to #1043 (closed), sources also don't need to be complete in remote execution but this requires more work to get functional.

Task description

• Add argument to source cache pull that only fetches the directory tree
• Figure out which scenarios we only want the directory tree and use this when pulling sources from a source cache
• Ensure that commands such as source checkout fail cleanly and maybe have a --pull option or similar
• Add tests for this.

Acceptance Criteria

With sources in a remote source cache, we should be able to build elements without having to pull source files locally.

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