Restore PATH override behavior

[bennyhat]

We took out PATH overrides awhile back. This breaks a few use cases.

Scenario: stub grep and then test something that uses it as a sub call. Ex. xargs grep cat

Xargs won't be able to find the grep override because it doesn't see the function override, and instead just does a PATH lookup

[bennyhat]

This will also help support things like stubbing commands with a space in the name. At present, bash 3.2, which we support, doesn't allow for spaces or newlines in function names (zsh and newer bashes should). This gap can be filled by actual files in the path