gnu: racket: Unbundle racket-minimal.

gnu: racket: Unbundle racket-minimal.

This change takes advantage of improved support for layered
and tethered installations in Racket 8.2.

* gnu/packages/racket.scm (extend-layer): New private variable.
This is a script for configuring a new config-tethered layer
chaining to an existing Racket installation.
* gnu/packages/racket.scm (racket)[source](snippet): Unbundle
`racket-minimal`.
[inputs]: Remove inputs that properly belong to `racket-minimal`.
[native-inputs]: Add `racket-minimal` and `extend-layer`.
[arguments]: Stop inheriting from `racket-minimal`. Add phase
'unpack-packages to move the sources and links file into place.
Replace 'configure phase using `extend-layer`.
Replace 'build phase using `raco setup`.
Delete 'install phase.
* gnu/packages/patches/racket-sh-via-rktio.patch: Rename to ...
* gnu/packages/patches/racket-minimal-sh-via-rktio.patch: ... this
file to placate `guix lint`.
* gnu/local.mk (dist_patch_DATA): Update accordingly.
* gnu/packages/racket.scm (racket-minimal)[source]: Likewise.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>