import: cran: Add support for git repositories.

* guix/import/cran.scm (vcs-file?): New procedure.
(download): Support downloading from git.
(fetch-description): Add a clause for the 'git repository type.
(files-match-pattern?): New procedure.
(tarball-files-match-pattern?): Implement in terms of FILES-MATCH-PATTERN?.
(directory-needs-fortran?, directory-needs-zlib?,
directory-needs-pkg-config?): New procedures.
(needs-fortran?, needs-zlib?, needs-pkg-config?): Rename these procedures...
(tarball-needs-fortran?, tarball-needs-zlib?, tarball-needs-pkg-config?):
...to this, and use them.
(file-hash): New procedure.
(description->package): Handle the 'git repository type.
* guix/import/utils.scm (package->definition): Handle package expression
inside of a let.
* guix/scripts/import.scm (guix-import): Handle let expressions.
* doc/guix.texi (Invoking guix import): Document it.