Add pytype kzip indexer tool sharing implementation with local indexer server.

The pytpe local indexer server already provides an interface to pytype
indexing which consumes compilation units and generates Kythe storage
entries.  Abstracting the implementation only slightly allows us to
create a tool for indexing compilation units from kzip archives
without duplicating any of the already-implemented and -tested logic
required for indexing compilation units generally.

PiperOrigin-RevId: 447457328

Author: Pytype Team

pytype/tools/xref/testdata/minimal.py

@@ -0,0 +1,5 @@
+# pylint: skip-file
+"""Minimal file contents to satisfy existence requirements."""
+
+# Minimal goal in order to define at least one goal.
+#- _.node/kind file