packages: 'find-packages-by-name' properly honors version prefixes.

Fixes <https://bugs.gnu.org/28446>.
Reported by Mark H Weaver <mhw@netris.org>.

* gnu/packages.scm (find-packages-by-name): Use 'version-prefix?'
instead of 'string-prefix?'.

gnu/packages.scm

@@ -188,7 +188,7 @@ (define find-packages-by-name
                             version>?)))
         (if version
             (filter (lambda (package)
-                      (string-prefix? version (package-version package)))
+                      (version-prefix? version (package-version package)))
                     matching)
             matching)))))