Speed up EnumeratePathsImpl in AggregateFileSystem

[garrynewman]

Simplify the behaviour of EnumeratePathsImpl in AggregateFileSystem by querying child filesystems with the same arguments instead of querying all all filesystems for every file and directory potentially 1000's of times.

In one query in our game, where we had an AggregateFileSystem as a child of another AggregateFileSystem this took it from 3.38 seconds to 0.16 seconds.

[xoofx]

The idea looks good (I really don't remember why I implemented it that way). The CI is failing on one test.

[xoofx]

So incidentally, I'm profiling an app that is making an heavy use of Aggregate FS and the internal lock is causing x2 times slower execution, so I might add more a bit more optimizations on top of your code. Though, I have probably a different use case (mainly relying on TryGetPath).

[garrynewman]

That fixes the broken test for me ❤

[xoofx]

Thanks!