Support nested .gitignore files in --pants-ignore-use-gitignore

[stuhood]

After #5682 and #18649, the situation is that Pants uses the root and global .gitignore files when --pants-ignore-use-gitignore is enabled (as it is by default).

This ticket covers supporting non-root/global .gitignore files in child directories.

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The implementation of --pants-ignore-use-gitignore initially used a naive recursive-upward parse of .gitignore files: this meant that for every file-ignore lookup, we would re-parse all parent .gitignore files. Consequently, that aspect was skipped in #9310.

Instead, in order to support nested .gitignore files efficiently while watching changes made to them, we would need to add recursive-upward memoization of .gitignore parsing. The memoization could either be accomplished by:

1. creating a new Gitignore node, which recursively depended upwards on itself to parse parent Gitignore nodes.
2. adding Gitignore information to the Scandir node, such that whenever we called Scandir on a directory, we also exposed and memoized a GitignoreStyleExcludes struct from the Node. Then, Scandir nodes would recursively depend on the parent Scandir node.

To reduce overhead (by creating fewer Nodes), I think that approach 2 would be cleaner. But if it ended up increasing complexity too much, then approach 1 would be fine as well.