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Targets act as aliases for their files (#10511)

### Problem

After extensive discussion, we think that in Pants 2.0, the definition of a target in a BUILD file will be approximately: "some metadata applied to a set of files, which can be used as an alias for those files". This is a fundamental change from the previous definition, which was something like: "a target is a collection of files with metadata that must always be built together".

But as described in #10455 and #10423, when "sub" (aka "file") targets are used inconsistently, it becomes necessary to worry about both:
1. whether a target builds as an atomic unit
2. whether each file in the target builds as an atomic unit

...whereas before file dependencies existed, only the former was necessary.

### Solution

Given the new definition, Pants ever operating on "the target as defined in the BUILD file" as the atomic unit no longer makes sense. Instead, for consistency: files should always be the new atomic unit (with batching applied only where it is possible to do so safely and transparently).

To accomplish this without also making significant changes to the `Target` API, this change adds expansion logic to the creation of `Targets` instances that replaces base targets with their corresponding file/sub targets. Additionally, base targets are adapted to always depend on their subtargets. This means that although all nodes in the build graph are still `Target` objects, post-expansion in `Targets`, each `Target` object will own either zero or one file.

Graph introspection goals mostly continue to operate directly on the targets as defined in BUILD files, and so an `UnexpandedTargets` type is introduced to be used in cases where the BUILD definitions should be used verbatim. Other cases where these are used include:
1. deleted files in `Owners`: a deleted file is matched against `UnexpandedTargets` to ensure that we detect what the owner "would have been" if the file had not be... (continued)

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