Missing melange tests

[anmonteiro]

This is meant to be a living list of tests I'd like to see for the melange integration before we document / ship it

• public library that depends on a private library, melange.emit stanza that depends on the public library
• include_subdirs + public libraries get correct --bs-package-output path
• test separate compilation (no need to recompile dependents if cmi didn't change)

[rizo]

Any chance #7104 could be considered before the official release? The described limitation currently prevents us from starting to incrementally adopt melange.

In particular, the described workflow with virtual libraries is essential for front-end/backend code sharing and compatibility with jsoo.

For instance, it would be convenient to define a virtual JSON-handling library with different backends for the native/byte mode and the melange mode.