Construct ClassIndex based on the CompilationInfo, to correctly reflect the available modules/classes.

[lahodaj]

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.

I think it is time to bite the bullet, at least partially, and try to fix this more conceptually.

The proposal herein is that we'll take ClassIndex from CompilationInfos, not ClasspathInfos, wherever possible, and that this ClassIndex will be configured based on the actual modules in the compilation.

(*) I am not quite certain we need module classpath (basically, classpath, but when modules are enabled). I think it would make more sense to only have classpath, but lets not dive into that too deep in this PR.

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