Improve error checking associated with cmake_source_dir skbuild.setup keyword

[jcfr]

This is related to the feature introduced in #168

• Raise an exception (or warning ?) if cmake_source_dir points to directory outside of the project source tree.
• When cmake_source_dir points to a sub directory with a CMakeLists.txt and there is already one in the top-level, report a warning indicating that the top-level one will be ignored.

[jcfr]

Raise an exception (or warning ?) if cmake_source_dir points to directory outside of the project source tree.

This is now supported since 5e52d5c fixing issue #335

[jcfr]

When cmake_source_dir points to a sub directory with a CMakeLists.txt and there is already one in the top-level, report a warning indicating that the top-level one will be ignored.

I don't think scikit-build should try to be too smart, for complex project is is reasonable to have such a situation.