patch 8.2.1426: Vim9: cannot call autoload function in :def function

Problem:    Vim9: cannot call autoload function in :def function.
Solution:   Load the autoload script. (closes #6690)
diff --git a/src/scriptfile.c b/src/scriptfile.c
index 3821591..320340f 100644
--- a/src/scriptfile.c
+++ b/src/scriptfile.c
@@ -1991,7 +1991,7 @@
     if (scriptname == NULL)
 	return NULL;
     STRCPY(scriptname, "autoload/");
-    STRCAT(scriptname, name);
+    STRCAT(scriptname, name[0] == 'g' && name[1] == ':' ? name + 2: name);
     for (p = scriptname + 9; (p = vim_strchr(p, AUTOLOAD_CHAR)) != NULL;
 								    q = p, ++p)
 	*p = '/';