[ExpUI] Register an iconMapper to override Python file type icon

Previously, the mapping file was put in a wrong place (not at the root of
a "resources" directory) and was not registered in any plugin descriptor.

However, under debug, the right icon was still displayed because pythonFile.svg
is also remapped in all-platform PlatformIconMappings.json. In actual installers
pythonFile.svg and PlatformIconMappings.json end up in different classloaders,
so this fallback doesn't work.

GitOrigin-RevId: ee361cd7d2d0158b0cc96bd652a5cb53bb7d88c5

python/python-psi-api/resources/META-INF/PythonPsi.xml

@@ -49,6 +49,7 @@
               hashBangs="python"
               implementationClass="com.jetbrains.python.PythonFileType"
               fieldName="INSTANCE"/>
+    <iconMapper mappingFile="PythonPsiApiIconMappings.json"/>
     <projectService serviceInterface="com.jetbrains.python.psi.types.TypeEvalContextCache"
                     serviceImplementation="com.jetbrains.python.psi.types.TypeEvalContextCacheImpl"/>
   </extensions>

python/python-psi-api/resources/PythonPsiApiIconMappings.json

@@ -5,7 +5,6 @@
         "python": {
           "expui": {
             "python.svg": "icons/com/jetbrains/python/pythonFile.svg"
-
           }
         }
       }