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

[Federico Tedin <federicotedin@gmail.com>]

>> I don't want to discourage you from continuing work on the filter
>> command by floating this idea.  Having filter commands and an isearch
>> option are not mutually exclusive.  On the contrary, I could see
>> myself wanting to use both.
>>
>> Perhaps it's better to regard this idea as separate from the "filter
>> by name" command for now.  Maybe I shouldn't have sidetracked this
>> issue by even raising it, but it's good to hear that you agree that
>> it's not a bad idea.
>
> I agree, let's first have a basic implementation of "filter by name",
> and then maybe add an isearch option later.  The way currently I have
> to search by name is to start isearch, type a name, and open occur
> from isearch.  Then jumping from the *Occur* buffer back to packages
> is so ugly workaround for having a proper filter by name.
> Filtering a huge list of packages will improve the current situation.
> (Another useful command would be "filter by description and name"
> like in Synaptic package manager.)

No problem then, I'll change the function names as Stefan suggested:

package-menu-search => package-menu-filter-by-name
package-menu-filter => package-menu-filter-by-keyword

Then, I'll add the new keybindings and also address the feedback he gave
me regarding my last patch. Thanks for the feedback!