Source `uninstall.fish` file before emitting event

Instead of having every plugin function inside the same file, which
increases the loading time unnecessarily and is not fishy, source the
`uninstall.fish` file for the plugin before the `uninstall_$plugin_name`
event is emitted.

pkg/omf/cli/omf.remove_package.fish

@@ -15,7 +15,7 @@ function omf.remove_package
   for path in {$OMF_PATH,$OMF_CONFIG}/{pkg}/$pkg
     not test -d $path; and continue
 
-    emit uninstall_$pkg
+    source $path/uninstall.fish; and emit uninstall_$pkg
     omf.bundle.remove "package" $pkg
 
     rm -rf $path