Use project-relative paths in sym-lib-table for imported libraries

When importing Altium projects, the MAIL_ADD_LOCAL_LIB handler stored
absolute paths in the sym-lib-table for native Altium library files
(.SchLib). This caused broken library references when the project was
moved to a different location.

Now library files outside the project directory are copied into it
during import, and the sym-lib-table URI uses ${KIPRJMOD}/ prefix
for project-relative paths. This matches the behavior of the Eagle,
CADSTAR, and EasyEDA Pro importers.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/22941

eeschema/cross-probing.cpp

@@ -46,6 +46,7 @@
 #include <libraries/symbol_library_adapter.h>
 #include <widgets/sch_design_block_pane.h>
 #include <widgets/kistatusbar.h>
+#include <wx/filefn.h>
 #include <wx/log.h>
 #include <trace_helpers.h>
 
@@ -856,6 +857,12 @@ void SCH_EDIT_FRAME::KiwayMailIn( KIWAY_MAIL_EVENT& mail )
         wxCHECK_RET( optTable.has_value(), "Could not load symbol lib table." );
         LIBRARY_TABLE* table = optTable.value();
 
+        wxString projectPath = Prj().GetProjectPath();
+
+        // First line of payload is the source project directory.
+        std::string srcProjDir;
+        std::getline( ss, srcProjDir, '\n' );
+
         std::vector<wxString> toLoad;
 
         while( std::getline( ss, file, '\n' ) )
@@ -875,11 +882,34 @@ void SCH_EDIT_FRAME::KiwayMailIn( KIWAY_MAIL_EVENT& mail )
 
             pi.reset( SCH_IO_MGR::FindPlugin( type ) );
 
+            wxString libTableUri;
+            bool     isProjectLocal = fn.GetFullPath().StartsWith( wxString( srcProjDir ) );
+
+            if( isProjectLocal )
+            {
+                // Project-local library: copy into the KiCad project directory and use a
+                // project-relative path so the sym-lib-table stays portable.
+                if( !fn.FileExists() )
+                    continue;
+
+                wxFileName projectFn( projectPath, fn.GetFullName() );
+
+                if( fn.GetFullPath() != projectFn.GetFullPath() && !projectFn.FileExists() )
+                    wxCopyFile( fn.GetFullPath(), projectFn.GetFullPath() );
+
+                libTableUri = wxS( "${KIPRJMOD}/" ) + fn.GetFullName();
+            }
+            else
+            {
+                // External library referenced by absolute path. Preserve the original path.
+                libTableUri = fn.GetFullPath();
+            }
+
             if( !table->HasRow( fn.GetName() ) )
             {
                 LIBRARY_TABLE_ROW& row = table->InsertRow();
                 row.SetNickname( fn.GetName() );
-                row.SetURI( fn.GetFullPath() );
+                row.SetURI( libTableUri );
                 row.SetType( SCH_IO_MGR::ShowType( type ) );
                 toLoad.emplace_back( fn.GetName() );
             }

kicad/import_proj.cpp

@@ -208,6 +208,10 @@ void IMPORT_PROJ_HELPER::addLocalLibraries( const std::set<wxString>& aNames, FR
 
     std::stringstream ss;
 
+    // First line is the source project directory so the handler can distinguish
+    // project-local libraries from external fixed-path references.
+    ss << TO_UTF8( m_InputFile.GetPath() ) << '\n';
+
     for( const wxString& name : aNames )
     {
         wxFileName fname( name );