wic: reimplement PluginMgr.get_plugin_methods

Simplified the implementation of get_plugin_methods:
- get rid of looping over the dicrtionary, used access by key instead
- get rid of filling a dictionary that passed as a parameter

(From OE-Core rev: 875d4eede61b548d64f426c2ef077cc17e50cd45)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/wic/plugins/imager/direct.py

@@ -198,11 +198,11 @@ class DirectPlugin(ImagerPlugin):
         disk_name = self.parts[0].disk
         if source_plugin:
             name = "do_install_disk"
-            methods = PluginMgr.get_source_plugin_methods(source_plugin,
-                                                          {name: None})
-            methods["do_install_disk"](self._image, disk_name, self, self.workdir,
-                                       self.oe_builddir, self.bootimg_dir,
-                                       self.kernel_dir, self.native_sysroot)
+            method = PluginMgr.get_plugin_methods('source', source_plugin,
+                                                   [name])[name]
+            method(self._image, disk_name, self, self.workdir,
+                   self.oe_builddir, self.bootimg_dir,
+                   self.kernel_dir, self.native_sysroot)
 
         full_path = self._image.path
         # Generate .bmap