Warn users when using a URL without an alias

Background

The use of aliases within URLs is strongly advised within BuildStream. It would be nice for this to be a warning provided to the user. This warning could configurable as fatal in a project for maintainers who wish to catch this sort of thing in CI.

Task description

• Detect when sources use URLs without aliases
• Collect URLs and provenances
• Provide a warning summarising these instances
• Revamp the changes from !627 (merged) to have warnings raised as exceptions by Context instead of Plugin. (As this summary of URLs will not be associated with a single plugin)

Acceptance Criteria

A user shall be provided with a warning when using URLs without aliases. These warnings shall be optionally fatal.

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