SI-7312 @deprecatedInheritance now ignores same-file subclasses

This allows us to deprecate external inheritances as a prelude
to sealing a class, without enduring the warnings ourselved in
interlude.

The `neg` test from SI-6162 has been split into a pos test (subclasses in the same compilation unit), and a neg test (subclasses in a different compilation unit.)

This will also silence these recently introduced spurious warnings (SI-7315):

```
qbin/scalac -deprecation src/library/scala/Tuple*.scala
src/library/scala/Tuple1.scala:19: warning: inheritance from class Tuple1 in package scala is deprecated: Tuples will be made final in a future version.
case class Tuple1[@specialized(Int, Long, Double) +T1](_1: T1)
```

Review by @JamesIry.