* gnu/ci.scm (channel-build-system, channel-source->package): Remove.
* gnu/packages/package-management.scm (channel-source->package): New
procedure, moved from (gnu ci).
* guix/build-system/channel.scm: New file, with code moved from (gnu ci).
* doc/guix.texi (Build Systems): Document it.
Until now, 'current-guix-package' was parameterized in the wrong
context. Thus, 'current-guix' would end up building a variant of the
'guix' package instead of the package returned by
'channel-source->package', which is much less expensive to build.
* etc/system-tests.scm (mparameterize): New macro.
(tests-for-current-guix): Change the 'value' field of each <system-test>
record to parameterize 'current-guix-package' for good.
The manifest can be passed to 'guix build -m', 'guix weather -m', and so
on. It can also be passed to an installed 'guix' (without
./pre-inst-env), with the exception so far of installation tests.
* build-aux/run-system-tests.scm: Remove. Move interesting bits move
to...
* etc/system-tests.scm: ... here. New file.
* Makefile.am (EXTRA_DIST): Remove 'build-aux/run-system-tests.scm' and
add 'etc/system-tests.scm'.
(check-system): Rewrite to run 'guix build -m etc/system-tests.scm'.
