Reproduce and fix class too large error on large java repo

[lihaoyi]

Converting https://github.com/iluwatar/java-design-patterns to Mill using #3756, we get a class too large error. Wrapping the Discover map keys in a proper anonymous class (rather than a lambda) seems to fix it. But I haven't managed to get a standalone repro on the problem, even with builds with 1000 modules, more than the link repo's count of 172

We really should to ensure we can reproduce the issue before merging the fix, so we can validate it works and continue working as part of our test suites