Feature Request: Improved target focusing API

The current target focusing API (i.e. xcodeproj.focused_targets and xcodeproj.unfocused_targets) allows for focusing or unfocusing any target you want. They can be annoying to use though if you want to (un)focus a category of targets, such as entire repositories. I say annoying instead of impossible because with a generated BUILD file a query can list out all of the targets in a repository. This has negative performance implications as well, passing around and interacting with large data structures.

I propose that we improve the target focusing API to allow specifying some "more than one target" patterns. The rough idea is to allow more than just labels in the {un,}ocused_targets attributes:

xcodeproj(
    …
    unfocused_targets = [
        "@repo1//some:target",
        unfocused_targets(type = focusing_type.repository, pattern = "repo2"),
        unfocused_targets(type = focusing_type.name.suffix, pattern = "_mocks"),
        unfocused_targets(type = focusing_type.package.suffix, pattern = "_UI"),
        unfocused_targets(type = focusing_type.package.component, pattern = "Tests"),
    ],
    …
)

I'm not sure on the actual spelling of the API (e.g. maybe `unfocused_targets.package.component("Tests") or something instead?), but the idea of being able to specify these matching clarifications is the key thing. With incremental generation mode there is a single location that can do these sorts of matches, making it a lot easier to add this sort of feature.