SCons: Fix uses of [].append instead of env.add_source_files()

Also added support for SCons project-absolute paths (starting with #) and
warning about duplicates in add_source_files(), and fixed
default_controller_mappings.gen.cpp being included twice after first build
due to *.cpp globbing.

Part of #30270.

editor/SCsub

@@ -31,7 +31,7 @@ if env['tools']:
     reg_exporters_inc = '#include "register_exporters.h"\n'
     reg_exporters = 'void register_exporters() {\n'
     for e in env.platform_exporters:
-        env.editor_sources.append("#platform/" + e + "/export/export.cpp")
+        env.add_source_files(env.editor_sources, "#platform/" + e + "/export/export.cpp")
         reg_exporters += '\tregister_' + e + '_exporter();\n'
         reg_exporters_inc += '#include "platform/' + e + '/export/export.h"\n'
     reg_exporters += '}\n'