The feature InternalImportsByDefault makes imports default to internal instead
of public. Applying the Swift 6 behavior of SE-0409 in Swift 5.
Let's use only that flag to track the Swift 6 behavior as well instead
of separately checking for the language version.
While use of access-level in imports is still gated by a flag for normal
source code, accept it in swiftinterfaces without the flag. This will
make it easier for older compilers to read newer swiftinterfaces created
by a future compiler where that restriction is lifted.
rdar://115455383
