bug#64358: “guix refresh” chokes on cran.scm

[Ricardo Wurmus <rekado@elephly.net>]

> Prior to that we see warnings like this:
>
> /home/rekado/dev/gx/branches/master/gnu/packages/cran.scm:36350:2: r-readtext: updating from version 0.82 to version 0.90...
> /home/rekado/dev/gx/branches/master/gnu/packages/cran.scm:36350:2: warning: r-readtext: no `version' field in source; skipping
>
> It’s as if the position in the file has been lost and it tries to update
> the definition of r-readtext that is no longer where the current port
> position is.

It seems that this is indeed the problem.  The value for a <package>’s
“location” field is known at compile/eval time and this value will not
be correct after the first substantial edit has taken place.

I see these options:

1 - pass a value to “update-package” that corresponds to line changes so
  far and let it return an updated value, making “update-package” aware
  of file changes.  This would be rather ugly.

2 - compute the location of the target package anew if the file it is
  located in has since been edited.

3 - never rely on the line number of the package location value; just open
  the specified file and always search it for a package definition.
  Optionally take the line number into account as a starting point for a
  search in both directions.

4 - integrate changes with git, so that all edits are commits that are
  applied to the very same base commit.

The third option seems the most reasonable and lightweight to me.

-- 
Ricardo