getdeps: add env subcommand

Summary:
X-link: https://github.com/facebookincubator/zstrong/pull/1025

Add getdeps.py `env` subcommand to get the environment from getdeps in a sourcable form so that its easier to run/test/debug the binaries

There is an existing `debug` subcommand, but it starts an interactive shell inside getdeps, which isn't so useful for programatic use.

To get the output clean enough to source I switched getdeps print() logging to go to stderr.

example usage:
```
$ (source <(./build/fbcode_builder/getdeps.py --allow-system-packages env mononoke_integration); which mononoke)
```

Reviewed By: jdelliot

Differential Revision: D64982397

fbshipit-source-id: 65212936d42185e4d395557b56d3dba499caa128

build/fbcode_builder/getdeps/runcmd.py

@@ -10,16 +10,12 @@ import select
 import subprocess
 import sys
 
+from shlex import quote as shellquote
+
 from .envfuncs import Env
 from .platform import is_windows
 
 
-try:
-    from shlex import quote as shellquote
-except ImportError:
-    from pipes import quote as shellquote
-
-
 class RunCommandError(Exception):
     pass