[declare] [tactics] Move declare to `vernac`

This PR moves `Declare` to `vernac` which will hopefully allow to
unify it with `DeclareDef` and avoid exposing entry internals.

There are many tradeoffs to be made as interface and placement of
tactics is far from clear; I've tried to reach a minimally invasive
compromise:

- moved leminv to `ltac_plugin`; this is unused in the core codebase
  and IMO for now it is the best place
- hook added for abstract; this should be cleaned up later
- hook added for scheme declaration; this should be cleaned up later
- separation of hints vernacular and "tactic" part should be also done
  later, for now I've introduced a `declareUctx` module to avoid being
  invasive there.

In particular this last point strongly suggest that for now, the best
place for `Class_tactics` would be also in `ltac`, but I've avoided
that for now too.

This partially supersedes #10951 for now and helps with #11492 .

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