Autogen: Revert changes to generate moc/rcc in subdirectories

Revert changes made for issue #12873 (closed). They regress existing builds that depend on the paths/symbols generated previously. Another approach will be needed to solve the name collision problem they were intended to solve. Leave the error diagnostics for the colliding cases that were added in the same topic as the above commits because they provide a useful early failure in relevant cases.

Fixes #16209 (closed).