Refactor yanked warning out of package finder

[uranusjr]

Noticed this when reading @pfmoore’s effort to wire pip internals to ResolveLib primitives.

The previous implementation assumes that when a best candidate is found, it will be downloaded/installed. This will no longer be the case for the new resolver, since it may decide to discard the candidate after inspection and fetch a new one.

By moving it to InstallRequirement.populate_link(), we can be sure that the warning is only set when the yanked link is actually populated for installation.

[uranusjr]

Ugh, this still seems to be the wrong place. We need to call prepare_linked_requirement() for dist metadata, but to make an InstallRequirement linked we need to call populate_link() first. So the warning needs to be triggered at the call site of populate_link(), not inside it.

[pradyunsg]

suggestion: add a check_yanked_link method to InstallRequirement, that looks are self.link.is_yanked and call it from within the current call site of populate_link (i.e. the resolver).

[uranusjr]

Replacing with #7801. The link.is_yanked strategy does not work because populate_link() actively replaces the link with cached location (losing the yanking context).