New resolver: Extras on direct install does not work

[uranusjr]

Context: #8146 (comment)

Some unorganised thoughts:

• This does not work because if a direct install is not available from indexes, the resolver throws ResolutionImpossible on that package because it cannot find any mat
• Maybe we can simply do ExplicitRequirement(self.base) here? Does this affect packages found on indexes?
• Another solution here would be to include the direct install as a candidate in find_matches. This means we need to resolve all direct references eagerly, and yield them at the right location (likely just after the already installed candidate). I think this actually makes sense?

[uranusjr]

This is fixed in #8291.