Update __init__.py with meca import

Co-Authored-By: Dongdong Tian <seisman.info@gmail.com>

Author: weiji14

pygmt/base_plotting.py

@@ -1581,4 +1581,4 @@ def text(
                 lib.call_module("text", arg_str)
 
     # GMT Supplementary modules
-    from pygmt.src.meca import meca  # pylint: disable=import-outside-toplevel
+    from pygmt.src import meca  # pylint: disable=import-outside-toplevel

pygmt/src/__init__.py

@@ -0,0 +1,5 @@
+"""
+Source code for PyGMT modules
+"""
+# pylint: disable=import-outside-toplevel
+from pygmt.src.meca import meca