Rewrite confusing stream API chain in SnippetMaps

After JDK-8263411, the stream call inside jdk.jshell.SnippetMaps::fullClassNameAndPackageToClass looks confusing:

        return Stream.concat(Stream.of("java.lang"), pkgs)
                     .filter(ipkg -> !ipkg.isEmpty() && ipkg.equals(pkg))
                     .map(ipkg -> full.substring(pkg.length() + 1))
                     .findFirst()
                     .orElse(full);

The `map` step ignores the input element, so the subsequent stream content doesn't depend on the previous stream content. This looks like a misuse of stream API. In fact, using anyMatch here would be better:

        return Stream.concat(Stream.of("java.lang"), pkgs)
                     .anyMatch(ipkg -> !ipkg.isEmpty() && ipkg.equals(pkg))
                     ? full.substring(pkg.length() + 1) : full;

Moreover, `!ipkg.isEmpty() && ipkg.equals(pkg)` is the same as `!pkg.isEmpty() && ipkg.equals(pkg)`, and as `!pkg.isEmpty()` check doesn't depend on the stream content, it could be extracted to a separate check. In case of empty package, stream is not necessary at all.