Figure out a plan to stabilize isolated extension usages

Isolated extension usages (`use_extension(..., isolate=True)`) are currently guarded behind the flag `--experimental_isolated_extension_usages`.

This feature is useful in particular as it allows an extension to easily generate repos scoped to a single user module. However, it is a rather big departure in the mental model of module extensions, as it essentially allows an extension to be evaluated "multiple times" (small print -- big simplification, etc etc); certain extensions that may rely on being "used" by its hosting module might break under this model; and the ID of an extension is not just ".bzl file label + extension name" anymore.

To stabilize this feature, we should evaluate other solutions to the "user-module-scoped repos" problem, and think about ways to mitigate the downsides of isolated extension usages (documentation? intentional limitations?).