Filter packages by name in list-packages. (Bug#36981)

Filter packages by name in list-packages. (Bug#36981)

* lisp/emacs-lisp/package.el (package-menu-filter-by-name): New
function to filter packages by name.
(package-menu-clear-filter): New function to clear applied filters.
(package-menu-filter-by-keyword): Rename function from
package-menu-filter.
(package-menu--generate): Don't change 'q' binding anymore.
(package-menu-mode-map): Bind '/ n' to package-menu-filter-by-name, '/
k' to package-menu-filter-by-keyword and '/ /' to
package-menu-clear-filter.
(package-menu-mode-menu): Update menu entries for the three functions.
* test/lisp/emacs-lisp/package-tests.el (package-test-list-filter-by-name)
(package-test-list-clear-filter): New tests.
* doc/emacs/package.texi: Document usage of
package-menu-filter-by-name, package-menu-clear-filter and update
reference to package-menu-filter-by-keyword.
* etc/NEWS: Announce changes.