guix build: Transformation options match packages by spec.

This allows us to combine several transformations on a given package, in
particular '--with-git-url' and '--with-branch'.

Previously transformations would ignore each other since they would all
take (specification->package SOURCE) as their replacement source,
compare it by identity, which doesn't work if a previous transformation
has already changed SOURCE.

* guix/scripts/build.scm (evaluate-replacement-specs): Adjust to produce
an alist as expected by 'package-input-rewriting/spec', with a package
spec as the first element of each pair.
(evaluate-git-replacement-specs): Likewise.
(transform-package-inputs):  Adjust accordingly and use
'package-input-rewriting/spec'.
(transform-package-inputs/graft): Likewise.
(transform-package-source-branch, transform-package-source-commit): Use
'package-input-rewriting/spec'.
(transform-package-source-git-url): Likewise, and adjust the
REPLACEMENTS alist accordingly.
(options->transformation): Iterate over OPTS instead of over
%TRANSFORMATIONS.  Invoke transformations one by one.
* tests/scripts-build.scm ("options->transformation, with-input"):
Adjust test to compare packages by name rather than by identity.
("options->transformation, with-git-url + with-branch"): New test.

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