source.py: Check for the availability of alias targets in preflight()

[gtristan]

Fixes #1860

[gtristan]

Looks like there are issues with calling Source.get_source_fetchers() in preflight... might be better off raising a lazy error here rather than a preflight error...

[gtristan]

Looks like there are issues with calling Source.get_source_fetchers() in preflight... might be better off raising a lazy error here rather than a preflight error...

Tried a lazy failing approach, that works but still wasn't satisfied with this.

Came back and did the check now in Source.mark_download_url(), verifying the correctness of this in the Source API contract.

Added a comment to SourceFetcher.mark_download_url() to reinforce the already existing contract, which already specified
that "This must be called for every URL in the configuration during Plugin.configure() if Source.translate_url() is not called."

This existing contract already validates the correctness of this approach to policing the alias, however it helps to reinforce the statement in the SourceFetcher docs and avoid misguiding plugin authors into thinking that SourceFetcher.mark_download_url() is a replacement of calling Source.mark_download_url()

Caveat: With this approach (including the previous approach), Buildstream will fail at load time in the case that user configuration specifies, for example, invalid tracking configuration (where aliases don't have specified URIs for tracking) when bst build or bst source fetch is called (i.e. it didn't need to be an error for the purpose of which bst was launched).

I think this is generally okay because you have to go out of your way to specify invalid tracking information anyway, so you shouldn't have invalid tracking configuration.

The current error in the test for this patch revision is:

fetch_source source at test.bst [line 3 column 2]: No fetch URI found for alias 'foo'

    Check fetch controls in your user configuration