add dependency type parameters to add_depends

add dependency type parameters to add_depends

Dependency types are represented by bits in the values of the %depends
and %depends_simple hashes.

Change the dependslist array saved to the index to a depends hash.
depends_simple is also converted from an array to a hash.

Note that the depends field used to be a string, and we still
have compat code to handle upgrades from that, as well as from the arrays.
I didn't use ikiwiki-transition because I don't want ikiwiki to break if
users forget to run it; also we're going to recommend a full rebuild on
upgrade to this version to get the improved dependency handling. So
this compat code can be removed or moved to ikiwiki-transition later.