pyoxidizer: add hooks to inject extra python packages and install files

We need this type of hook to inject our internal extension and resource files
at Google. Presumably this could be useful to others, so instead of trying to
carry an internal patch we've done this in a modular way that should be of
value upstream.

I'm extremely puzzled by the behavior of glob() on Windows, and I'll
be filing at least one (probably two) bugs upstream about it.

Differential Revision: https://phab.mercurial-scm.org/D11092

rust/hgcli/pyoxidizer.bzl

@@ -103,6 +103,12 b' def make_exe(dist):'
         exe.add_python_resources(
             exe.pip_install(["-r", ROOT + "/contrib/packaging/requirements-windows-py3.txt"]),
         )
+    extra_packages = VARS.get("extra_py_packages", "")
+    if extra_packages:
+        for extra in extra_packages.split(","):
+            extra_src, pkgs = extra.split("=")
+            pkgs = pkgs.split(":")
+            exe.add_python_resources(exe.read_package_root(extra_src, pkgs))
 
     return exe
 
@@ -144,6 +150,17 b' def make_windows_install_layout(manifest'
         print("copying %s to %s" % (path, new_path))
         manifest.add_file(manifest.get_file(path), path = new_path)
 
+    extra_install_files = VARS.get("extra_install_files", "")
+    if extra_install_files:
+        for extra in extra_install_files.split(","):
+            print("adding extra files from %s" % extra)
+            # TODO: I expected a ** glob to work, but it didn't.
+            #
+            # TODO: I know this has forward-slash paths. As far as I can tell,
+            # backslashes don't ever match glob() expansions in 
+            # tugger-starlark, even on Windows.
+            manifest.add_manifest(glob(include=[extra + "/*/*"], strip_prefix=extra+"/"))
+
     # We also install a handful of additional files.
     EXTRA_CONTRIB_FILES = [
         "bash_completion",