search: Rename search_providers to open-search-providers

We will allow applications to hook into shell's search by registering
a service which implements a well-known DBus interface.
"search-providers" is a reasonable directory name for applications to
drop their registration files, but it conflicts with "search_providers"
used by open search providers - rename the latter to avoid confusion.

https://bugzilla.gnome.org/show_bug.cgi?id=663125