.bat or .exe files can be considered executable on Windows. This PR
expands the regex for detectable packages to allow for the detection
of packages that vendor .bat wrappers (intel mpi for example).
Additional changes:
* Outside of Windows, when searching for executables `path_hints=None`
was used to indicate that default path hints should be provided,
and `[]` was taken to mean that no defaults should be chosen
(in that case, nothing is searched); behavior on Windows has
now been updated to match.
* Above logic for handling of `path_hints=[]` has also been extended
to library search (for both Linux and Windows).
* All exceptions for external packages were documented as timeout
errors: this commit adds a distinction for other types of errors
in warning messages to the user.
069762cd37