host-perl-xml-parser: rename and refactor with perl infrastructure

Even though libxml-parser-perl had a Config.in file with an option to
enable it on the target, this option was hidden by a dependency on
BR2_HOST_ONLY. So in practice, it was not possible to enable
libxml-parser-perl on the target. This allows us to rename
libxml-parser-perl to perl-xml-parser to follow the new naming
convention of Perl packages, without having to introduce
Config.in.legacy material.

In addition to this rename, the package is converted to use the newly
introduced Perl package infrastructure.

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>