`__precompile__(false)` in extensions is not applied correctly

[topolarity]

It looks like we're not properly modeling for __precompile__(false) in extensions:

Although, in some scenarios it leads to

│  ┌ Error: Error during loading of extension FlexiMapsDictionariesExt of UnionCollections, use `Base.retry_load_extensions()` to retry.
│  │   exception =
│  │    1-element ExceptionStack:
│  │    Error when precompiling module, potentially caused by a __precompile__(false) declaration in the module.

...

Originally posted by @aplavin in #56204

We should either:

1. ban __precompile__(false) in extensions
2. handle __precompile__(false) specially in extensions so that it is non-viral (which means updating loading to avoid loading this extension during pre-compilation)
3. handle __precompile__(false) the same way we always do - make it viral and exclude all loading packages from pre-compilation

My vote is (1) or (2), because (3) seems very surprising and goes against the grain of "optional functionality" that extensions generally provide.