Move old-style metadata facilities to `deprecated`

[mih]

Protocol of plan made in devcall:

Move remaining metadata commands to -deprecated

• metadata
• aggregate:-metadata
• search

Announce this plan now (informally), and release the move with the next major release.

With that announcement, we announce that metalad provides the next generation metadata handling for datalad.

There is no requirement for metalad to provide an API-compatible replacement for these commands. Rather, it is appreciated for them to stay intact and usable in parallel with -deprecated installed.

[mih]

https://twitter.com/datalad/status/1567222620570927105?t=GJ9r-VL9ll-BQ-8nzk8sJQ&s=19

[christian-monch]

I will take this one. Plan:

1. extend datalad-deprecated with the metadata-commands
2. test with datalad branch https://github.com/mih/datalad/tree/mnt-metadata-mv

[yarikoptic-gitmate]

Issue fixed in 0.18.0