Allow to replace built-in features

[Enrico Zini]

(from a conversation with @DonKult)

Is there actually a way to prevent staticsite from load say "tags" – so that I may load "tags-ng"?

I've been considering implementing something like a features_exclude
setting, but so far I haven't had a need for it so I procrastinated it.
You give me a good reason to have it.

Also, depending on a feature in the deny list should in theory just
ignore the dependency, or possibly disable loading of the depending
feature? In your case, however, you'd like to have tags-ng run instead
of tags, and then series would integrate with tags-ng instead of tags,
so I guess that disabling series if tags is blacklisted wouldn't work
for you.

As a note: This imagined tags-ng does not exist nor do I actually plan
for one. It does indeed poses a set of interesting design possibilities…
In theory a tags-ng would need a way to say "I am like tags" (aka
Provides: tags), but that leads eventually also to all the other dependency
relations and a full-blown resolver…

There could be a possible shortcut here: features define the name they take in the feature database, as such:

FEATURES = {
    "tags": TaxonomyFeature,
}

It could be possible to say that if two features register themselves with the same name, the last one wins and the previous one get ignored.

When loading features, one can postpone instantiating the feature classes until both the built-in and the theme features have been loaded, and only then build the dependency graph, do the topological sorting, and instantiate feature classes.