Modules: Do not implicitly add new functions via old Check Modules

The conversion of Check<Lang>CompilerFlag, SourceCompiles, and
SourceRuns over to the new functions has the possibility of breaking
projects that had functions with those existing names.

To reduce the possibility of collisions we now have all the
legacy code call functions that start with `cmake_`, and users
will need to explicitly include the new modules to get the
non-prefixed versions

Fixes: #21359