import: crate: Add ‘--lockfile’ option.

* guix/import/crate.scm (cargo-inputs-from-lockfile)
find-cargo-inputs-location, extract-cargo-inputs): New procedures.
* guix/scripts/import/crate.scm (%options): Add ‘--lockfile’ option.
(show-help): Add it.
(guix-import-crate): Use it.
* doc/guix.texi (Invoking guix import): Document it.

Change-Id: I291478e04adf9f2df0bf216425a5e8aeba0bedd9

doc/guix.texi

@@ -14957,6 +14957,11 @@ version instead instead of aborting.
 If a crate dependency is not (yet) packaged, make the corresponding
 input in @code{#:cargo-inputs} or @code{#:cargo-development-inputs} into
 a comment.
+@item --lockfile=@var{file}
+@itemx -f @var{file}
+When @option{--lockfile} is specified, the importer will ignore other options
+and won't output package expressions, instead importing source expressions
+from @var{file}, a @file{Cargo.lock} file.
 @end table
 
 @item elm