link hook returning a Module: timing

[jorendorff]

If a link hook returns a Module, do we commit that to the registry immediately, even though other modules in the same linkage unit may not have loaded yet?

[jorendorff]

@samth and @jorendorff say: Wait until a LinkSet including this module gets linked and committed.

I'll try implementing this.

[samth]

Clarifying: this means that the result of a module factory function is not committed until it's linked with the modules that depend on it. (The other alternative would be to commit immediately after the factor function returns.)