* guix/derivations.scm (build-derivations): Accept <derivation-input>
records among DERIVATIONS.
* tests/derivations.scm ("build-derivations with specific output"): Test
it.
Consequently, the whole graph of <derivation> object is readily
available without having to go through 'read-derivation-from-file',
which could have cache misses if the requested <derivation> object had
been GC'd in the meantime. This is an important property for the
performance of things like 'derivation-build-plan' that traverse the
derivation graph.
* guix/derivations.scm (<derivation-input>): Replace 'path' field by
'derivation'.
(derivation-input-path): Adjust accordingly.
(derivation-input-key): New procedure.
(derivation-input-output-paths): Adjust accordingly.
(coalesce-duplicate-inputs): Likewise.
(derivation-prerequisites): Use 'derivation-input-key' to compute keys
for INPUT-SET.
(derivation-build-plan): Likewise.
(read-derivation): Add optional 'read-derivation-from-file' parameter.
[make-input-drvs]: Call it.
(write-derivation)[write-input]: Adjust to new <derivation-input>.
(derivation/masked-inputs): Likewise, and remove redundant
'coalesce-duplicate-inputs' call.
(derivation)[input->derivation-input]: Change to consider only the
derivation case. Update call to 'make-derivation-input'.
[input->source]: New procedure.
Separate sources from inputs.
(map-derivation): Adjust to new <derivation-input>.
* tests/derivations.scm ("parse & export"): Pass a second argument to
'read-derivation'.
("build-expression->derivation and derivation-prerequisites")
("derivation-prerequisites and valid-derivation-input?"): Adjust to new
<derivation-input>.
* guix/ui.scm (show-what-to-build)[build-or-substitutable?]: Remove.
Use 'derivation-build-plan' instead of
'derivation-prerequisites-to-build', passing it all of DRV at once, and
remove 'fold2' shenanigans and postprocessing of BUILD.
The new 'derivation-build-plan' procedure has a more appropriate
signature: it takes a list of <derivation-inputs> instead of taking one
<derivation>. Its body is also much simpler.
* guix/derivations.scm (derivation-build-plan): New procedure.
(derivation-prerequisites-to-build): Express in terms of
'derivation-build-plan' and mark as deprecated.
* tests/derivations.scm: Change 'derivation-prerequisites-to-build'
tests to 'derivation-build-plan' and adjust accordingly.
* guix/ui.scm (display-search-results): New procedure.
* guix/scripts/package.scm (find-packages-by-description): Remove
'unzip2' call and return a list of pairs.
(process-query): Change to use 'display-search-results'.
* guix/scripts/system/search.scm (find-service-types): Remove 'unzip2'
call and return a list of pairs.
(guix-system-search): Use 'display-search-results'.
* guix/build/syscalls.scm (terminal-dimension): New procedure.
(terminal-columns): Rewrite in terms of 'terminal-dimension'.
(terminal-rows): New procedure.
* tests/syscalls.scm ("terminal-rows"): New test.
