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.py

@@ -923,6 +923,27 @@ class DebugCmd(ProjectCmdBase):
         self.create_builder(loader, manifest).debug(reconfigure=False)
 
 
+@cmd(
+    "env",
+    "print the environment in a shell sourceable format",
+)
+class EnvCmd(ProjectCmdBase):
+    def setup_project_cmd_parser(self, parser):
+        parser.add_argument(
+            "--os-type",
+            help="Filter to just this OS type to run",
+            choices=["linux", "darwin", "windows"],
+            action="store",
+            dest="ostype",
+            default=None,
+        )
+
+    def run_project_cmd(self, args, loader, manifest):
+        if args.ostype:
+            loader.build_opts.host_type.ostype = args.ostype
+        self.create_builder(loader, manifest).printenv(reconfigure=False)
+
+
 @cmd("generate-github-actions", "generate a GitHub actions configuration")
 class GenerateGitHubActionsCmd(ProjectCmdBase):
     RUN_ON_ALL = """ [push, pull_request]"""