Display __all__ on module pages

[abesto]

https://github.com/zsol/py.wtf/pull/33 added explicit exports to the index; let's show them! Probably as a dedicated section near the top of the module contents page.

[zsol]

33 is not enough, we'll need proper xrefs even for intra-linked symbols here unfortunately (as described in #8), because for this

from foo import bar as baz
__all__ = ["baz"]

We'll want to generate a link that's called baz, but points to the symbol foo.bar. Can't do that with plain strings.

[zsol]

65 should be all that's needed from the indexer side 😄

[zsol]

Fixed by #79