Tell find_package() to warn if called without an explicit mode.

If find_package() is called without an explicit mode option (MODULE, CONFIG, or NO_MODULE) and no Find<pkg>.cmake module is in CMAKE_MODULE_PATH then CMake implicitly assumes that the caller intends to search for a package configuration file. If no package configuration file is found then the wording of the failure message must account for both the case that the package is really missing and the case that the project has a bug and failed to provide the intended Find module. If instead the caller specifies an explicit mode option then the failure message can be more specific.

Set CMAKE_FIND_PACKAGE_WARN_NO_MODULE to TRUE to tell find_package() to warn when it implicitly assumes Config mode. This helps developers enforce use of an explicit mode in all calls to find_package() within a project.