crate_universe: Enable modext isolation.

This should allow us to build our python and ruby
code independently - in particular, we can now do shallow
checkouts of one without the other.
Previously, the modext introduced cross-dependency.
This also reduces the amount of work we do in the
crate universe processing for the other language, even
though it's unused.

This does need renaming the module, as otherwise
the generated paths from rules_rust get too long
for Windows :(

misc/bazel/internal/zipmerge/zipmerge_test.cpp

@@ -75,7 +75,7 @@ std::string get_file(const char* name) {
     return ret;
   }();
   // this works from both `codeql` and the internal repository
-  for (auto prefix : {"_main", "codeql~"}) {
+  for (auto prefix : {"_main", "ql~"}) {
     auto ret = runfiles->Rlocation(prefix + "/misc/bazel/internal/zipmerge/test-files/"s + name);
     if (fs::exists(ret)) {
       return ret;