Previously, if a source file was referenced from multiple packages, it
just became owned by the first one AstGen happened to reach; this was a
problem, because it could lead to inconsistent behaviour in the compiler
based on a race condition. This could be fixed by just analyzing such
files multiple times - however, it was pointed out by Andrew that it
might make more sense to enforce files being part of at most a single
package. Having a file in multiple packages would not only impact
compile times (due to Sema having to run multiple times on potentially a
lot of code) but is also a confusing anti-pattern which more often than
not is a mistake on the part of the user.

Resolves: #13662