wpkg's packages.xsd specifies that we should order elements as

<install><condition /><exit /></install>

rather than

<install><exit /><condition /></install>

I don't believe there's any need for both mainversion and patchversion; the
Chemistry package has always used just one without any problems.

Having the mst in a dir involving %mainversion% will lead to a maintenance
headache when doing upgrades. I've been using the same mst without problems
for a few years' worth of DC versions.

Automatic initial import of acrobatreaderdc.xml from git@gitlab.developers.cam.ac.uk:ch/co/wpkg/wpkg-packages

Tidied up the XML: we don't need to apply each and every patch, as
just the lastest one is sufficient.