Adds "element-provides" support to element dependencies

An element can define a list of elements provided by itself. This allows
using an alternative element and keeping element dependencies at the
same time. This will be needed for example for mariadb element which will be
an alternative to mysql element - mysql dependency can be kept everywhere
but user can choose mariadb explicitly as a replacement by putting "mysql"
into "element-provides" file.

Change-Id: I7f193612f279dd71d9f1f1edfc60009838a00c5a
Merge "Adds "element-provides" support to element dependencies"