The overriding pairs cursor used to detect erased signature clashes
was turning a blind eye to any pair that contained a private method.

However, this could lead to a `VerifyError` or `IllegalAccessError`.

Checking against javac's behaviour in both directions:

```
% cat sandbox/Test.java
public abstract class Test {
  class C { int foo() { return 0; } }
  class D extends C { private <A> int foo() { return 1; } }
}

% javac sandbox/Test.java
sandbox/Test.java:3: error: name clash: <A>foo() in Test.D and foo() in Test.C have the same erasure, yet neither overrides the other
  class D extends C { private <A> int foo() { return 1; } }
                                      ^
  where A is a type-variable:
    A extends Object declared in method <A>foo()
1 error
```

```
% cat sandbox/Test.java
public abstract class Test {
  class C { private int foo() { return 0; } }
  class D extends C { <A> int foo() { return 1; } }
}

% javac sandbox/Test.java
%
```

This commit only the exludes private symbols from the superclass
from the checks by moving the test from `excludes` to `matches`.