Deprecate and remove spawn.find_executable

[jaraco]

After doing some more refactoring, I've learned that find_executable is very similar to shutil.which (similar enough that shutil.which was probably copy-pasted from find_executable). shutil.which had evolved PATHEXT searching, so the change in #248 brings the distutils behavior more inline with what shutil.which is doing.

My instinct - distutils should deprecate find_executable and in spawn utilize shutil.which.

Originally posted by @jaraco in #248 (comment)