Display emerge search results incrementally (412471)

This makes emerge --search / --searchdesc display individual search
results as soon as they become available. A search._iter_search method
is split out from the search.execute method, and the search.output
method operates on that. The spinner is now disabled, but the spinner
code remains, in case we later decide to enable it optionally.

X-Gentoo-Bug: 412471
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=412471