Warn if a module being referenced is using an experimental language version

If an 'experimental' module is published to the registry, it may be consumed by someone without realizing it was created with an experimental language version.

This may result in the consumer being unexpectedly broken (since we reserve the right to make breaking changes to experimental features).

The issue names no files, tests, or entry points; begin by locating the registry module-reference handling and the language-version metadata path. Done means consumers receive a warning when a referenced published module uses an experimental language version.

Written by the indexing model from the issue text.