Importable python_lib.zip assets

Lots of plumbing to allow an asset named python_lib.zip to be imported
by jailed Python code.

This function can find the "python_lib.zip" asset, and is passed down
through ModuleSystem and LoncapaSystem so that capa problems have access
to the zipfile.

common/test/data/uploads/python_lib_zip/README.txt

@@ -0,0 +1,3 @@
+Use this directory to make the python_lib.zip file above:
+
+$ zip -r python_lib.zip python_lib_zip/*

common/test/data/uploads/python_lib_zip/number_helpers.py

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+def seventeen():
+    return 17
+
+def fortytwo(x):
+    return 42+x

common/test/data/uploads/python_lib_zip/sub/submodule.py

@@ -0,0 +1 @@
+HELLO = "world"