Do not fetch file blobs for build-only dependencies with remote execution

Jürg Billeterrequested to merge
juerg/partial-cas into master

Changes proposed in this merge request:

  • Mandate all elements/plugins to support virtual directories when remote execution is configured (drop fallback to local builds)
  • Consider artifacts with missing file blobs as cached when remote execution is configured and the element is not a runtime dependency of the build target
  • Skip pulling file blobs from artifact cache when remote execution is configured and the element is not a runtime dependency of the build target
  • Skip fetching output files from remote execution CAS server if the element is not a runtime dependency of the build target and the files are not required for artifact pushing

Part of #832 (closed).

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