Require virtual directory support for element plugins

Element plugins can currently directly access the sandbox directory on the local filesystem with Sandbox.get_directory() if the plugin doesn't have the BST_VIRTUAL_DIRECTORY flag. However, this doesn't work with buildbox-run or remote execution. As the aim is to drop the current bwrap sandboxing backend in favor of the buildbox-run sandboxing backend, we need to drop support for plugins that don't work with virtual directories.

• Extend the Directory API
  • !1827 (merged)
  • !1878 (merged)
• Port bst-plugins-experimental and freedesktop-sdk plugins to Sandbox.get_virtual_directory(). Test freedesktop-sdk build.
  • bst-plugins-experimental!89
  • bst-plugins-experimental!96
  • bst-plugins-experimental!97
  • bst-plugins-experimental!98
  • bst-plugins-experimental!99
• !1887 (merged) Drop Sandbox.get_directory() method, allow_real_directory parameter and BST_VIRTUAL_DIRECTORY flag.