bug#36981: 26.2; request: add searching by package name to list-packages

[ndame]

When installing a package from list-packages it's often
inconvenient to find a package which has a string which occurs
many times in the packages buffer. (e.g. in the descriptions).

The package buffer has f for filtering, but it filters only for
package keywords, not names.

Add a new command s for search which searches (or filters) the
package list by a substring match on the package name.