Remove .bat and .exe from commands where possible.

We expect .BAT and .EXE to be on PATHEXT. Also, although Python
CreateProcess doesn't honor PATHEXT, it will try .EXE. This allows most
commands to be platform-independent, and also resolves the issue with
the cpython package not including python.bat.

No-Tree-Checks: true
Docs-Preview: https://skia.org/?cl=135626
Bug: chromium:852581
Change-Id: Iab4189407df44ff4ad4d37da07ff52414229d397
Reviewed-on: https://skia-review.googlesource.com/135626
Commit-Queue: Ben Wagner <benjaminwagner@google.com>
Auto-Submit: Ben Wagner <benjaminwagner@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
12 files changed