1. Avoid forcing info of non-Scala interface members

This avoids parsing the ostensibly malformed class definitions that
correspond to a Groovy lambda defined in an interface.

2. Be more tolerant of absent inner classfiles

Taking a leaf out of javac's book (see transcript below),
we can use stub symbols for InnerClass entries that don't
have corresponding class files on the compilation classpath.

This will limit failures to code that directly refers to the
inner class, rather than any code that simply refers to the
enclosing class.

It seems that groovyc has a habit of emitting incongrous
bytecode in this regard. But this change seems generally
useful.

```
% cat sandbox/{Test,Client}.java
public class Test {
	public class Inner {}
}
public class Client {
	public Test.Inner x() { return null; }
}

% javac -d . sandbox/Test.java && javac -classpath . sandbox/Client.java

% javac -d . sandbox/Test.java && rm 'Test$Inner.class' && javac -classpath . sandbox/Client.java
sandbox/Client.java:2: error: cannot access Inner
	public Test.Inner x() { return null; }
	           ^
  class file for Test$Inner not found
1 error

% cat sandbox/{Test,Client}.java
public class Test {
	public class Inner {}
}
public class Client {
	public Test.NeverExisted x() { return null; }
}

% javac -classpath . sandbox/Client.java
sandbox/Client.java:2: error: cannot find symbol
	public Test.NeverExisted x() { return null; }
	           ^
  symbol:   class NeverExisted
  location: class Test
1 error

% cat sandbox/{Test,Client}.java
public class Test {
	public class Inner {}
}
public class Client {
	public Test x() { return null; }
}
topic/groovy-interop ~/code/scala2 javac -d . sandbox/Test.java && rm 'Test$Inner.class' && javac -classpath . sandbox/Client.java # allowed
```