pyoxidizer: allow extensions to be loaded from the file system

It seems that setting this config is all that's needed to be able to
load extensions from the file system (which we clearly want). Thanks
for making this work, Gregory Szorc!.

Differential Revision: https://phab.mercurial-scm.org/D8122

contrib/packaging/pyoxidizer.bzl

@@ -21,6 +21,8 b' def make_exe():'
     config = PythonInterpreterConfig(
         raw_allocator = "system",
         run_eval = code,
+        # We want to let the user load extensions from the file system
+        filesystem_importer = True,
         # We need this to make resourceutil happy, since it looks for sys.frozen.
         sys_frozen = True,
         legacy_windows_stdio = True,