The url: of a element should not be part of the cache-key calculation if the ref: is specified

Update: I've updated the title and tags of this issue to reflect that it is currently a documentation task.

Steps to reproduce

• Build a element from a specific URL
• Move the tarball to another server
• Change the .bst file to point to the new location (ref: will be the same)
• Buildstream will rebuild

What is the current bug behavior?

• Rebuild not needed

What is the expected correct behavior?

• Not need to rebuild if the ref: is exactly the same

Relevant logs and/or screenshots

• This happen in freedesktop-sdk when chaning bzip2 to another mirror (same ref:): https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/jobs/88004717

Possible fixes

If ref: is present, do not include the url in the cache calculation at in _downloadablefilesource.py