FindClassResolutionsOperation.findValidPackagesContainingSimpleType

Reverted findValidPackagesContainingSimpleType() to search only in
existing Java workspace projects and not in the entire platform.
Searching in the entire platform kills the (currently synchronous)
content assist by freezing UI.

Change-Id: Ib6a885b98c4652ddca71c2ee3f32c6c49e77e721
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>