bug#70689: guix search doesn't weigh word matches higher than subword matches

[Richard Sent <richard@freakingpenguin.com>]

Hi Guix!

When running guix search, relevance in synopsis and description fields
are computed strictly by the number of matches, both as a word and as a
subword. Ideally, if a search string matches an isolated word in a
search, that result should be considered more relevant than simply
matching a subword, even multiple times.

To illustrate, imagine trying to find what package provides the `rsh`
binary and running running `$ guix search rsh`. This binary is part of
`inetutils` and the description field contains:

> Inetutils is a collection of common network programs, such as an ftp
> client and server, a telnet client and server, an rsh client and
> server, and hostname.

Most likely, this is what the user is interested in. However, inetutils
does not show up until roughly the ~75th result with a relevance of 2
(the lowest possible relevance).

Almost every search result beforehand contains the string "rsh" as a
component of another word, such as "marshaling", "powershell", and
"hershey". However, these match multiple times and are weighted
significantly higher.

Ideally, guix search should rate inetutils higher because the string
"rsh" occurs as its own word, not as a component of another, unrelated
word. (Very, very people would search "rsh" looking for matches with
"hershey", even if "hershey" occurs multiple times.)

Another example of where this can happen is with "dig", part of the bind
package. Searching for "dig" returns garbage because "dig" is a common
subword. Bind is scored with a relevance of 2, even though bind's
description emphasises that dig is part of it.

This would improve the experience when searching with strings that
commonly occur as subwords.

Since this change can't occur in a vacuum, care should be taken not to
reduce the effectiveness of other reasonably forseeable search queries.

-- 
Take it easy,
Richard Sent
Making my computer weirder one commit at a time.