Underscore Handling Fixes (#47581)
Previously to this PR, if an absolute path contained an `_` it would
ignore all app files. If for example the Next.js app was in a folder
named `/some/_directory` then today:
```
/some/_directory/app/page.js <- should 200, currently 404
/some/_directory/app/route.js <- should 200, currently 404
/some/_directory/app/_internal/route.js <- should 404, currently 200 in development
```
This also enables the same `_` functionality for app routes in
development. After this PR, we get:
```
/some/_directory/app/page.js <- 200
/some/_directory/app/route.js <- 200
/some/_directory/app/_internal/route.js <- 404
```
