buildstream/plugins/sources/_downloadablefilesource.py: Store etag along with cache.

Tristan Van Berkomrequested to merge
tristan/etag-mr-fixup into master

Fixes #377 (closed).

This merge request replaces !441 (closed) with some changes:

  • Place private methods at the end of the file, not the beginning.
  • Dont include history of changes in the branch, this branch included fixes made for review comments separately instead of submitting a single coherent patchset (in this case, only one commit anyway).

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