Finds given binary and returns it's full path.
Throws ExecutableNotFound if the binary was not found.
Search strategy: "/" in the name of the binary means that given executable_name is a path so it's returned as-is (wrapped in Path).
If PATH environment variable exists, the given directories are searched.
If PATH is not set built-in value for PATH is used to search: /usr/local/bin:/usr/bin:/bin:/sbin:/usr/sbin
Source: stdlib.ngs:2733