[Sema] Stop filtering nested protocol types from lookup
This check wasn't ever correct, because the fact that the the protocol comes
from another module doesn't change the fact that the type is valid for lookup
within this module. It incorrectly rejects the following, valid code:
```swift
// In A.swift
public protocol A {}
```
```
// In B.swift
import A
extension A {
typealias B = Int
func b(_ b: Self.B) {}
}
```
