Emit warning if public include path is unversioned

[TheLartians]

One thing I actually do worry about now is if a user installs different versions of the same package without using a versioned include path. Then the include path would only contain the most recent installed version, but the CMake modules would still exist for all versions.

Perhaps we could check if the include path contains the version string and emit an informative warning if the flag NO_VERSION_SUFFIX isn't set. (using flag would completely overwrite the old versions, so CMake won't find old versions at all)

Originally posted by @TheLartians in https://github.com/TheLartians/PackageProject.cmake/pull/15#discussion_r573721426