cmake/FindLibpsl: protect against pkg-config "half-detection"

[vszakats]

Same issue as seen before with libssh2: libpsl's pkg-config module
depends on another module, but that's not found. CMake ends up reporting
LIBPSL_FOUND=YES, while leaving LIBPSL_INCLUDE_DIRS empty. Then
the build fails to find psl.h.

The missing dependency in this case is icu4c, which is "keg-only",
meaning it's not exposed in the default Homebrew header, pkg-config,
lib, etc locations. It must be added to the PKG_CONFIG_PATH env, as
suggested by the warnings messages of pkgconf.

To avoid this fallout, let's ensure that LIBPSL_INCLUDE_DIRS is
non-empty when detecting via pkg-config and fall back to the CMake
detection method otherwise.

This was an issue till Homebrew libpsl 0.21.5_1, fixed in 0.21.5_2, that
no longer depends on icu4c.

Example log:

-- Checking for module 'libpsl'
--   Found libpsl, version 0.21.5
Package icu-uc was not found in the pkg-config search path.
Perhaps you should add the directory containing `icu-uc.pc'
to the PKG_CONFIG_PATH environment variable
Package 'icu-uc', required by 'libpsl', not found
[...]
-- Found Libpsl (via pkg-config):  (found version "0.21.5")
[...]
In file included from curl/_bld/lib/CMakeFiles/libcurl_static.dir/Unity/unity_0_c.c:4:
In file included from curl/lib/altsvc.c:32:
In file included from curl/lib/urldata.h:145:
curl/lib/psl.h:28:10: fatal error: 'libpsl.h' file not found
         ^~~~~~~~~~
1 error generated.

Follow-up to 39c741b #15408