import: crate: Use existing package satisfying semver requirement.

import: crate: Use existing package satisfying semver requirement.

If a package satisfying the dependency's semver requirement already exists,
use it. Prior to this change the highest version matching the semver
requirement was used (and imported in case it was not defined as package
already).

When resolving a dependency (now done in `sort-map-dependencies`), first
search for a package matching the semver requirement and only if this fails
reach out for a crate.

* guix/import/crate.scm (crate->guix-package)[find-package-version]: New
  function. [dependency-name+version]: New function.
  [sort-map-dependencies]: Use it instead of lambda function.

* tests/crate.scm (test-doctool-crate, test-doctool-dependencies): New
  variables.
  ("self-test …", "cargo-recursive-import-hoors-existing-packages"): New
  tests.