make packagers life easier

[Tomáš Čech]

Hello Guix,

I'd like to share some thoughts about packaging to find I'm doing it
wrong or show that there is room for improvements. I don't think that
the proposed improvements are hard to implement (from algorithmic POV)
and I'm aware that all the things can be solved on my side, but that
is not the point.

Stop after/before phase
=======================

What it can help with?

- download sources of package without constructing URL
- prepare sources for creating or applying patch
- when preparing configure parameters you can stop right after
 configuration phase

How to implement?
- inject always failing phase into phases list

Another level of this feature could be even continuing of the build.


Ommit developed package from generations
========================================

How it can help?

When I work on packages I have to take care of handling generations
even when I don't care. It complicates removing old package builds
using `guix gc -d' and it is unwanted burden.

How to implement?
I'm not exactly sure, but not creating new generation with the package
could be sufficient.



Improve output of the build
===========================

What I miss?

Mostly one particular and obvious thing - after successful build print
the path of newly created store in well visible, easy-to-mouse and
easy-to-parse way.

example:

New store created: /gnu/store/s0meb0r1ngandl000ngh4shlead1ngt0-package-1.0/

Right now I need to dig that from any build message above.



And in future it would be nice to add some colors to output. It helps
with orientation speed a lot.

Thanks for reading,

S_W