FindPostgreSQL: Fix regression in computation of library directory

Since commit 368bcba28a (FindPostgreSQL: Find debug lib, imported
configs, 2019-05-09, v3.15.0-rc1~93^2) the `PostgreSQL_LIBRARY` variable
may contain a list and therefore should not be passed to the
`get_filename_component` command directly.  Instead spell out the logic
to compute `PostgreSQL_LIBRARY_DIR` from one of the per-configuration
variables.

Fixes: #19444

Modules/FindPostgreSQL.cmake

@@ -172,14 +172,21 @@ endfunction()
 # any PostgreSQL_LIBRARY that is already specified and skip the search.
 if(PostgreSQL_LIBRARY)
   set(PostgreSQL_LIBRARIES "${PostgreSQL_LIBRARY}")
+  get_filename_component(PostgreSQL_LIBRARY_DIR "${PostgreSQL_LIBRARY}" PATH)
 else()
   __postgresql_find_library(PostgreSQL_LIBRARY_RELEASE ${PostgreSQL_LIBRARY_TO_FIND})
   __postgresql_find_library(PostgreSQL_LIBRARY_DEBUG ${PostgreSQL_LIBRARY_TO_FIND}d)
   include(${CMAKE_CURRENT_LIST_DIR}/SelectLibraryConfigurations.cmake)
   select_library_configurations(PostgreSQL)
   mark_as_advanced(PostgreSQL_LIBRARY_RELEASE PostgreSQL_LIBRARY_DEBUG)
+  if(PostgreSQL_LIBRARY_RELEASE)
+    get_filename_component(PostgreSQL_LIBRARY_DIR "${PostgreSQL_LIBRARY_RELEASE}" PATH)
+  elseif(PostgreSQL_LIBRARY_DEBUG)
+    get_filename_component(PostgreSQL_LIBRARY_DIR "${PostgreSQL_LIBRARY_DEBUG}" PATH)
+  else()
+    set(PostgreSQL_LIBRARY_DIR "")
+  endif()
 endif()
-get_filename_component(PostgreSQL_LIBRARY_DIR ${PostgreSQL_LIBRARY} PATH)
 
 if (PostgreSQL_INCLUDE_DIR)
   # Some platforms include multiple pg_config.hs for multi-lib configurations