bug#20255: 'search-paths' should respect both user and system profile.

[宋文武]

> [...]
>>
>> The idea to generate profile from search-paths is not new,
>> I heard it from you IIRC.
>> I think it's the time to do it.
>
> Agreed, the plan makes sense and I think we have all the bits.
>
> A related question is whether to encode search path environment
> variables into the manifest (currently they are “guessed” by looking at
> same-named packages; see (guix build package).)  I think that would
> probably simplify things and make it easier to share this environment
> variable code.
>
> Thoughts?
I see, currently search-paths depends on the packages recipes. If we
update the related scheme code, then search-paths got updated, even we
didn't touch packages in profile at all.  It's a little confusing.
So I think we should encode the search-paths for each package in
manifest.
> Thanks,
> Ludo’.