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>.
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