search: add --fmt-stdin

search: add --fmt-stdin

When --fmt-stdin is set, JSON is taken directly from standard input,
instead of from aur-query. This allows in particular to pipe aur-pkglist
--info / --search output to aur-search.

For example, the following pipeline returns all orphan packages whose
name starts in 'r-':

 $ aur pkglist --info | jq -r '{
        "results": [.[] |
            select(.Maintainer == null) |
            select(.Name | test("^r-"))
        ], "type": "search", "version": 5
    }' | aur search --fmt-stdin

This example originates from aurweb returning "Too many results" when
querying an empty maintainer (orphan) search. See:

https://gitlab.archlinux.org/archlinux/aurweb/-/issues/357