This PR fixes an issue where an @source pointing to a file in a nested folder was not scanned when a later @source pointed to a file in a parent folder.

E.g.:

@source "./nested/index.html";
@source "./index.html";

When using @source pointing to a specific file, then we want to make sure that we ignore other files since they are not listed explicitly. To ensure that these patterns don't read other files, we inject a * ignore pattern before it. You can think of the above being expanded to:

Ignored { base: "/project/src/nested", pattern: "*" }
Pattern { base: "/project/src/nested", pattern: "/index.html" }
Ignored { base: "/project/src", pattern: "*" }
Pattern { base: "/project/src", pattern: "/index.html" }

The problem with this is that the Ignored { base: "/project/src", pattern: "*" } pattern results in ignoring the nested folder as well. This means that we never even walk into the nested folder, so the earlier @source "./nested/index.html" never matches anything.

We could switch the order in user land, but that's going to be hard to maintain (and order matters for undoing/redoing earlier rules, so we can't re-order internally either). Instead, we can scope the ignore pattern to the current path only, and not deeply nested. In other words, the pattern should become:

- *
+ /*

It's a very subtle difference, but the pattern from above will now become:

  Ignored { base: "/project/src/nested", pattern: "*" }
  Pattern { base: "/project/src/nested", pattern: "/index.html" }
- Ignored { base: "/project/src", pattern: "*" }
+ Ignored { base: "/project/src", pattern: "/*" }
  Pattern { base: "/project/src", pattern: "/index.html" }

We already do this when an unrestricted root (e.g. @source "./nested") lives inside the base of a restricted pattern. This works because every source base is also its own walk root, and a /* pattern only matches direct children so it can't ignore anything when walking from the nested root itself.

This PR extends that same check to restricted pattern bases: if another @source pattern has its base nested inside the current base, we emit /* instead of *.

Note that we only relax the pattern to /* when such a nested root actually exists. Sibling folders that no @source points into (e.g. an ignore-me folder next to nested) are still direct children, so they still match /* and are never walked.

Fixes: #20333

Test plan

1. Added a regression test with the reproduction setup
2. Added a similar test with another sibling folder that should still be ignored
3. Tested it against the actual reproduction:

Before:

After:

Notice that the text-green-500 now appears as expected.