Tab completion should only list plugins with flags

Currently, tab completion lists all registered plugin names, even if they don't use maktaba and don't respect maktaba flags. Since pull req google/maktaba#39, this can include plugins that were detected on rtp but weren't explicitly registered, which can bloat Glaive's plugin list with plugins that can't actually be meaningfully configured by Glaive.

We should try to pare down the list of plugins to only include ones that make sense for Glaive. This is a little more complicated than it looks since we can't just check whether a plugin defines flags: most plugins have at least one flag, the `plugin[]` flag maktaba automatically defines. One approach would be to check `PLUGIN.AddonInfo()` to see whether the plugin defines a dependency on maktaba, since right now it's not really possible for plugins to query flags without calling maktaba functions.