patch 8.2.4134: MS-Windows: test for import with absolute path fails

Problem:    MS-Windows: test for import with absolute path fails.
Solution:   Handle path starting with slash as an absolute path.
diff --git a/src/version.c b/src/version.c
index 1e9ab2d..8ac5c08 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4134,
+/**/
     4133,
 /**/
     4132,
diff --git a/src/vim9script.c b/src/vim9script.c
index 41df6bf..b356347 100644
--- a/src/vim9script.c
+++ b/src/vim9script.c
@@ -468,7 +468,13 @@
 	    vim_free(from_name);
 	}
     }
-    else if (mch_isFullName(tv.vval.v_string))
+    else if (mch_isFullName(tv.vval.v_string)
+#ifdef BACKSLASH_IN_FILENAME
+	    // On MS-Windows omitting the drive is still handled like an
+	    // absolute path, not using 'runtimepath'.
+	    || *tv.vval.v_string == '/' || *tv.vval.v_string == '\\'
+#endif
+	    )
     {
 	// Absolute path: "/tmp/name.vim"
 	if (is_autoload)