Rewrite rlocation function in fish

[Matthew-Benson]

Today, we package a standalone bash script for rlocation support for runfiles.

This also leaves us with a weird, hidden dependency on bash, which could cause issues stemming from system bash not being hermetic.

Unfortunately, the rlocation function we're getting from bash rules actually doesn't seem to work right in some cases with bzlmod. We should instead copy the logic from the Go library runfiles in our implementation.