### Problem
Python requires packages to have an `__init__.py` file to be recognized as a proper module for the sake of imports.

#7696 does this for Pytest, but inlines the logic, even though it will likely be helpful for other Python rules as well.

Further, because this logic was originally written before being able to from `Digest->Snapshot` thanks to #7725, we had to use `FilesContent` to grab the paths of the digest. This would mean that every single source file would be materialized and persisted to memory, resulting in extremely high memory usage (found as part of investigation in #7741). There is no need for the actual content, just the paths, so this is a huge inefficiency.

Will close #7715.

### Solution
Generalize into `@rule(InitInjectedDigest, [Snapshot])`, where `InitInjectedDigest` is a thin wrapper around a `Digest`.

We take a `Snapshot` because we need the file paths to work properly. This contrasts with earlier using `FileContents` to get the same paths. A `Snapshot` is much more light weight.

We return a `Digest` because that is the minimum information necessary to work properly, and the caller of the rule can then convert that `Digest` back into a `Snapshot`.

### Result
It will now be easier for other Python rules to work with Python packages.

The unnecessary memory usage is now fixed. The V2 Pytest runner now has a space complexity of `O(t   t*e   b)`, rather than `O(t   t*e   s)`, where `t` is # targets, `e` is # env vars, `b` is # `BUILD` files, and `s` is # source files.