Extracting the source tarball multiple times is very slow and a
speedup of >2x (without network I/O) can be achieved by coalescing all
NEEDS-X? functions into a single one, which extracts a tarball only once.
* guix/import/cran.scm (tarball-needs-fortran?): Remove unused function.
(needs-fortran?): Ditto.
(tarball-files-match-pattern?): Ditto.
(tarball-needs-zlib?): Ditto.
(needs-zlib?): Ditto.
(tarball-needs-pkg-config?): Ditto.
(needs-pkg-config?): Ditto.
(source-dir->dependencies): New function.
(source->dependencies): New function.
(description->package): Use it.
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