Don't expose Block modules from hir

[matklad]

hir::Module has source, which returnsModuleSource, which stores BlockExpr as a possible variant. treating block with items as module might be fine, but we definitely should not expose this complexity to an IDE. Not sure whats the best way to achieve that though.

One approach is to add a hir::ModuleSource which asserts that the module is real, and sped the rest of our lives tracking assertion failures.

Another approach is to somehow distinguish real and fake modules in the compiler level by either turning a module into an enum, or by introducing a separate ModuleScope.

[matklad]

Tentatively assigning @jonas-schievink here, as he definitely knows better what's the best solution here.

[jonas-schievink]

When converting ModuleId to Module, we could turn block modules into the containing "real" module, then whatever ModuleSource we use doesn't have to account for block expressions. Whether that's the right solution depends on what Module is used for, which I'm not totally sure about.

[flodiebold]

I think trying to hide this would end badly. IMO it'd be better to expose it with an enum, and then have methods to jump to the containing 'real' module.