Construct ClassIndex also based on module paths, so that types on module paths can be found as well.

[lahodaj]

This is an alternative to #9543, after discussing this problem with @dbalek.

Looking at Jaroslav's report:
https://lists.apache.org/thread/f5st2t0wspbdmjt6v5ojobzqpl7cjk9n

There's a general problem with ClassIndex - it is based on ClasspathInfo only, and only on boot, compile and source path. This means the ClassIndex does not see things on the module path, and module classpath(*). ComputeImports has a hack to workaround this, but code completion does not.

The simplified proposal here simply enhances the ClasspathInfo.getClassIndex() so that the ClassIndex is based on module paths. This means the ClassIndex will start to return symbols that are not resolvable in the context of the current class. Which the clients can filter out.

Compared to #9543, this is much easier, and does not require all clients to migrate to a different method.

As a last caveat, this approach does not allow to reflect --patch-module in the ClassIndex, so index-based feature won't work for symbols from patch modules.

What do you think?

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