Since most of the find modules use the `<PackageName>_FOUND` result
variables, this now also syncs it for the FindPkgConfig module. The
`PkgConfig_FOUND` result variable is available since CMake 3.3 and
contains the same value. There is also `PKGCONFIG_FOUND` result variable
automatically set with the same value but for simplicity isn't
documented. The uppercased `<PACKAGENAME>_FOUND` result variables set by
find modules are also considered legacy variables.
This displays the library name in the result message instead of the
include directory. In cases where the include directory is /usr/include
is a bit less informative compared to a library name. For example,
/usr/lib/x86_64-linux-gnu/libsqlite3.so
Since commit d74210a8bd (CMP0017: Remove support for OLD behavior,
2024-11-17) we can rely on CMP0017's NEW behavior unconditionally.
Calling `include(FindPackageHandleStandardArgs)` in a builtin module
will always get the builtin `FindPackageHandleStandardArgs`.
