make check-includes does not check out of tree builds correctly

I configure tor to build with an out-of-tree build in tor/build-c.

When I run "make check-includes", it checks tor/build-c/src. But it should check tor/src.

I'm marking this as a sponsor 31 roadmap item, because we need to get this check working, so we know that #29217 (moved) creates new modules correctly.