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 :(

.bazelrc

@@ -24,5 +24,6 @@ common --registry=file:///%workspace%/misc/bazel/registry
 common --registry=https://bcr.bazel.build
 
 common --@rules_dotnet//dotnet/settings:strict_deps=false
+common --experimental_isolated_extension_usages
 
 try-import %workspace%/local.bazelrc