Fix bad import of Rigify legacy modules

There is an issue with the construction of module names. Since there is no
top-level module "legacy", the `"legacy.%s.%s" % (RIG_DIR, rig_type)`
construct fails.

Without these fixes, a unittest fails:

Start testing: May 23 12:19 CEST
----------------------------------------------------------
3/18 Testing: script_load_modules
3/18 Test: script_load_modules
Command: "/home/sybren/workspace/blender-git/build_linux/bin/blender" "--background" "-noaudio" "--factory-startup" "--env-system-scripts" "/home/sybren/workspace/blender-git/blender/release/scripts" "--python" "/home/sybren/workspace/blender-git/blender/tests/python/bl_load_py_modules.py"
Directory: /home/sybren/workspace/blender-git/build_linux/tests/python
"script_load_modules" start time: May 23 12:19 CEST
Output:
----------------------------------------------------------
Traceback (most recent call last):
  File "/home/sybren/workspace/blender-git/blender/tests/python/bl_load_py_modules.py", line 231, in <module>
    main()
  File "/home/sybren/workspace/blender-git/blender/tests/python/bl_load_py_modules.py", line 226, in main
    load_modules()
  File "/home/sybren/workspace/blender-git/blender/tests/python/bl_load_py_modules.py", line 178, in load_modules
    __import__(mod_name_full)
  File "/home/sybren/workspace/blender-git/build_linux/bin/release/scripts/addons/rigify/legacy/__init__.py", line 42, in <module>
    from . import utils, rig_lists, generate, ui, metarig_menu
  File "/home/sybren/workspace/blender-git/build_linux/bin/release/scripts/addons/rigify/legacy/rig_lists.py", line 76, in <module>
    rig_list = get_rig_list("")
  File "/home/sybren/workspace/blender-git/build_linux/bin/release/scripts/addons/rigify/legacy/rig_lists.py", line 47, in get_rig_list
    rig = utils.get_rig_type(module_name)
  File "/home/sybren/workspace/blender-git/build_linux/bin/release/scripts/addons/rigify/legacy/utils.py", line 702, in get_rig_type
    submod = importlib.import_module(name, package=MODULE_NAME)
  File "/usr/lib/python3.5/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
ImportError: No module named 'legacy'

Warning! '/home/sybren/.config/blender/2.78/scripts/startup/pillarsdk/__init__.py' has no register function, this is now a requirement for registerable scripts
archimesh: Imported multifiles
measureit: Imported multifiles
Paths:
        '/home/sybren/workspace/blender-git/build_linux/bin/release/scripts/modules'
        '/home/sybren/workspace/blender-git/build_linux/bin/release/scripts'
        '/home/sybren/.config/blender/2.78/scripts'
MeasureIt: Cleaning data
++++++++++++++++++++++++++++++
MeasureIt: Cleaning data
Error: Not freed memory blocks: 1, total unfreed memory 0.000214 MB
<end of output>
Test time =   0.87 sec
----------------------------------------------------------
Test Failed.
"script_load_modules" end time: May 23 12:19 CEST
"script_load_modules" time elapsed: 00:00:00
----------------------------------------------------------

End testing: May 23 12:19 CEST

rigify/legacy/utils.py

@@ -698,7 +698,7 @@ def copy_attributes(a, b):
 def get_rig_type(rig_type):
     """ Fetches a rig module by name, and returns it.
     """
-    name = "legacy.%s.%s" % (RIG_DIR, rig_type)
+    name = "rigify.legacy.%s.%s" % (RIG_DIR, rig_type)
     submod = importlib.import_module(name, package=MODULE_NAME)
     importlib.reload(submod)
     return submod
@@ -707,7 +707,7 @@ def get_rig_type(rig_type):
 def get_metarig_module(metarig_name):
     """ Fetches a rig module by name, and returns it.
     """
-    name = "legacy.%s.%s" % (METARIG_DIR, metarig_name)
+    name = "rigify.legacy.%s.%s" % (METARIG_DIR, metarig_name)
     submod = importlib.import_module(name, package=MODULE_NAME)
     importlib.reload(submod)
     return submod