bootstrap: Clarify name of configured source directory

Rename CMAKE_ROOT_DIR to CMAKE_BOOTSTRAP_SOURCE_DIR to clarify both that
it is only for bootstrap and that it refers to the source directory.
bootstrap: Fix "make test" and "make package" targets (#14989)

Since commit v3.0.0-rc1~374^2 (Refactor internal resource location APIs
and initialization, 2013-11-07) a bootstrap-built "cmake" tries to
reference "ctest" and "cpack" executables next to itself, which never
exist.  Teach cmSystemTools::FindCMakeResources, when bootstrap-built,
to refer to the "ctest" and "cpack" executables in the location where
they will be built after "make".
Merge topic 'bootstrap-cmake-locations' into next

6650b0fa bootstrap: Fix "make test" and "make package" targets (#14989)
bc151757 bootstrap: Clarify name of configured source directory