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/manifest.py

@@ -8,6 +8,7 @@
 import configparser
 import io
 import os
+import sys
 from typing import List
 
 from .builder import (
@@ -391,7 +392,10 @@ class ManifestParser(object):
             return False
         for key in envs:
             val = os.environ.get(key, None)
-            print(f"Testing ENV[{key}]: {repr(val)}")
+            print(
+                f"Testing ENV[{key}]: {repr(val)}",
+                file=sys.stderr,
+            )
             if val is None:
                 return False
             if len(val) == 0: