Every time we call is_exe_in_path(), it must re-scan the $PATH.

It's not a big performance issue, but we do look up some of the same

binaries more than once.
This change addresses this by allowing is_exe_in_path() to take a

cache parameter, which is a dictionary matching names to found paths

(or None, if not found). It also prevents a lookup if the path

provided is an absolute path.
Really, though, this is laying some groundwork for some improved

dependency checking, where the cache will be reused for looking up

binary files at both dependency resolution and process execution time.

Unit tests passed.
Made use of this in the upcoming dependency change.