import: crate: Trim version for names after left-most non-zero part.

import: crate: Trim version for names after left-most non-zero part.

This complies to how versions are matched for caret requirements in crates:
An update is allowed if the new version number does not modify the left-most
non-zero digit in the major, minor, patch grouping.

* guix/import/crate.scm (version->semver-prefix): New function.
  (make-crate-sexp)[format-inputs]: Use it.
  (make-crate-sexp): Use it to pass shorter version to package->definition.
* guix/import/utils.scm (package->definition): Change optional parameter
  APPEND-VERSION? into APPEND-VERSION?/STRING. If it is a string, append its
  value to name.
* tests/crate.scm: Adjust tests accordingly.