vs*.generate-solution shouldn't be inherited by submodules.
Fix GUIDs of VS solution folders to be unique.

Submodules with the same name (e.g. 'tests' under several different
components) would incorrectly generate their GUID for .sln file from
just the short, non-unique name. This was incorrect and resulted in
confusing VS with what it regarded as duplicate solution folders.

The unfortunate side-effect of this fix is that it changes all the
generate GUID values, but it can't be avoided.