Load symbol library cache when creating a new cache.

Do not rely on buffering always being disabled to ensure the library gets
loaded when the cache didn't exist.

Use wxFileName::GetModificationTime() rather than trying to use our own
TimestampDir() to get a single library file modification time.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/22191

eeschema/sch_io/kicad_sexpr/sch_io_kicad_sexpr.cpp

@@ -1603,11 +1603,16 @@ void SCH_IO_KICAD_SEXPR::cacheLib( const wxString& aLibraryFileName,
 
     if( !m_cache || !m_cache->IsFile( aLibraryFileName ) || m_cache->IsFileChanged() )
     {
+        bool isNewCache = false;
+
+        if( !m_cache )
+            isNewCache = true;
+
         // a spectacular episode in memory management:
         delete m_cache;
         m_cache = new SCH_IO_KICAD_SEXPR_LIB_CACHE( aLibraryFileName );
 
-        if( !isBuffering( aProperties ) )
+        if( !isBuffering( aProperties ) || isNewCache )
             m_cache->Load();
     }
 }

eeschema/sch_io/sch_io_lib_cache.cpp

@@ -30,6 +30,7 @@ SCH_IO_LIB_CACHE::SCH_IO_LIB_CACHE( const wxString& aFullPathAndFileName ) :
     m_modHash( 1 ),
     m_fileName( aFullPathAndFileName ),
     m_libFileName( aFullPathAndFileName ),
+    m_fileModTime( 0 ),
     m_isWritable( true ),
     m_isModified( false )
 {
@@ -72,14 +73,14 @@ long long SCH_IO_LIB_CACHE::GetLibModificationTime()
     if( !fn.IsDir() )
     {
         m_isWritable = fn.IsFileWritable();
+        return fn.GetModificationTime().GetValue().GetValue();
     }
     else
     {
         m_isWritable = fn.IsDirWritable();
         wildcard = wxS( "*." ) + wxString( FILEEXT::KiCadSymbolLibFileExtension );
+        return TimestampDir( fn.GetPath(), wildcard );
     }
-
-    return TimestampDir( fn.GetPath(), wildcard );
 }
 
 
@@ -97,7 +98,7 @@ bool SCH_IO_LIB_CACHE::IsFileChanged() const
         return false;
 
     if( !fn.IsDir() && fn.IsFileReadable() )
-        return TimestampDir( fn.GetPath(), fn.GetFullName() ) != m_fileModTime;
+        return fn.GetModificationTime().GetValue().GetValue() != m_fileModTime;
 
     if( fn.IsDir() && fn.IsDirReadable() )
         return TimestampDir( fn.GetPath(),