don't try to use vendored crates in non-fbsource builds

Summary:
Watchman's vagrant images rely on mapping a watchman source directory
fetched from fbsource into the VM. Thus, they sometimes look like
fbsource, and that's largely okay. However, the vendored cargo support
breaks. Instead, explicitly skip vendored cargo when there is no
fbsource dir.

Reviewed By: xavierd

Differential Revision: D36386674

fbshipit-source-id: 61f2af19507fecd2342cfc94bbb7120ab91c33b4

build/fbcode_builder/getdeps/builder.py

@@ -12,17 +12,20 @@ import shutil
 import stat
 import subprocess
 import sys
+import typing
 
 from .dyndeps import create_dyn_dep_munger
 from .envfuncs import add_path_entry, Env, path_search
 from .fetcher import copy_if_different
 from .runcmd import run_cmd
 
+if typing.TYPE_CHECKING:
+    from .buildopts import BuildOptions
 
 class BuilderBase(object):
     def __init__(
         self,
-        build_opts,
+        build_opts: "BuildOptions",
         ctx,
         manifest,
         src_dir,