Planned setup.ini changes for early 2018

[Jon Turney]

* Add depends: to version descriptions

This is a version-specific list of required packages (as opposed to 
requires:, which is per-package, and contains the union of the 
dependencies for all versions).

I believe that historical setup versions will either ignore, or can 
handle depends: (just containing package names, without version 
relations) relatively sanely (see [1] et seq. for details).

* De-duplicate source archives

Source archives which are identical[2] between x86 and x86_64 will be 
moved to paths starting src/ in the release area.

Doing post-hoc de-duplication is unfortunate, but worthwhile given the 
potential size saving in mirrors (see [3] et seq.), until cygport can be 
taught how to make suitable source packages (which has several 
unresolved issues, also discussed at [3], [4] et seq.).

* Migrate from setup.hint to pvr.hint in release area

I also plan to migrate all remaining setup.hints to pvr.hints in the 
release area (setup.hint in uploads have been automatically migrated 
since [5]).

This should have no effect on the generated setup.ini, but enables some 
complexity (some of which isn't implemented properly, see [6]) to be 
removed from calm.

[1] https://cygwin.com/ml/cygwin-apps/2017-12/msg00020.html
[2] my current implementation considers two archives identical if they 
have an identical set of members, with identical contents. Permitted 
differences include the mtime, mode or owner of members.
[3] https://cygwin.com/ml/cygwin-apps/2017-04/msg00069.html
[4] https://cygwin.com/ml/cygwin-apps/2017-05/msg00012.html
[5] https://cygwin.com/ml/cygwin-apps/2017-11/msg00044.html
[6] https://cygwin.com/ml/cygwin-apps/2017-10/msg00123.html