Ensure we use the `cmake` corresponding to the running `cpack`
even if it is not first in `PATH` or has had its name changed.
This was accidentally left out in commit v3.7.0-rc1~81^2 (CPack/RPM:
Generate source rpm (SRPM) packages on demand, 2016-09-19).
These modules are not meant to be included by user code, they are
only an internal implementation detail for CPack. Having them live
in the main Modules directory with documentation was misleading, so
they have been moved into Modules/Internal/CPack, and their
documentation has been stripped following its move into the new
"CPack Generators" section. No-op modules which contained only
documentation have been removed entirely.
The only module that hasn't been moved is CPackIFW, because it
contains user-facing macros which would be lost if it were moved.
So, the CPackIFW module has been updated with a note explaining what
needs to (eventually) happen.
