WIP: Add `--no-fetch` flag to subcommands

Description

This introduces a new flag --no-fetch that was mentioned in the mailing list and raised here.

Proposed changes

To put simply, this adds a new flag, --no-fetch, to explicitly disable fetching for elements, including junctions.

• bst {pull,push,track,checkout} element(s) --no-fetch command

  If the element(s) depend on junction(s), they won't be fetched automatically and error out.

• bst {workspace-open,workspace-reset,source-bundle} element --no-fetch .. command

  Similar to above for junctions + fetching of element is disabled no matter --track is supplied or not.

This merge request, when approved, will close: N/A at the moment.

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