0.4.2 release has an XML module defined globally

[al2o3cr]

The 0.4.2 release includes code that defines a top-level XML module (from #86), which causes compilation to fail when loaded in a project that also defines a top-level XML module (or a warning, depending on the setting of --warnings-as-errors).

Usual practice would be to move that module to Elixlsx.XML and add alias lines to the couple of places that use it.