Currently, all folders and files in the current tree are turned into
IndexablePath, and then excluded files are filtered out after.

When there are large file trees that are meant to be excluded, this
results in a lot of unnecessary work.

ActiveStorage stores files in the `tmp` directory in many many small
folders. So does Bootsnap. Additionally, node_modules can become quite
large. Ruby LSP has to traverse all of these files,
even though the entire directory should just be ignored.

Instead we can skip any top-level directories whose paths have been
excluded.

We still need to loop through all IndexablePath objects compare them to the
exclude_patterns, in case nested folders or file name patterns were excluded.
Still, skipping some large top-level directories proves to be a big
performance improvement.

Before this PR in my Rails app, `indexables` took 76 seconds to run. Now
it takes, 0.17 seconds. Before and after code both return the same exact
file list.