Print license information on '--xml --meta'

The nixpkgs manual prescribes the use of values from stdenv.lib.licenses
for the meta.license attribute. Those values are attribute sets and
currently skipped when running nix-env with '--xml --meta'. This has the
consequence that also nixpkgs-lint will report missing licenses.

With this commit nix-env with '--xml --meta' will print all attributes
of an attribute set that are of type tString. For example the output for
the package nixpkgs.hello is

    <meta name="license" type="strings">
      <string type="url" value="http://spdx.org/licenses/GPL-3.0+" />
      <string type="shortName" value="gpl3Plus" />
      <string type="fullName" value="GNU General Public License v3.0 or later" />
      <string type="spdxId" value="GPL-3.0+" />
    </meta>

This commit fixes nixpkgs-lint, too.

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