project-vc: Provide default implementation of `project-list-files`

Rework the implementation of `project-files<vc>` so as to
delegate more to the `project-list-files` operation, including
the default implementation.

This can change the behavior in some cases, because it changes
which ignore settings apply where and also it can now use
a dedicated VC backend implementation even for directories
that are outside of the project's root.  But the effect should
hopefully be marginal and rather positive by making the behavior
more "regular".

* lisp/progmodes/project.el (project-vc--backend): New function.
(project-files) <vc>: Use it.  Call VC's `project-list-files` operation
for all dirs with a backend and remove the `vc-not-supported` fallback.
Inline the `project--dir-ignores` call.
(vc-default-project-list-files): New function, extracted from
`project-files<vc>`.
(vc-git-project-list-files): Use it instead of signaling `vc-not-supported`.
(project--vc-ignores): New function, extracted from `project-ignores<vc>`.
Remove the `vc-not-supported` since the operation has a default
implementation.  Use DIR's `ignore-completion-table` regardless
if we're at the root of the project.
(project-ignores) <vc>: Use it.

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