packaging: make "mercurial.defaultrc" a key in package_data
Before this patch, we had a `mercurial` key with a `defaultrc/*.rc`
value. It seems more correct to have a `mercurial.defaultrc` key with
a `*.rc` value since `mercurial.defaultrc` it became a pacakge in
1390bb81163e (help: get helptext/ data from `resources` module if
available, 2019-12-12).
Differential Revision: https://phab.mercurial-scm.org/D8856