[tex-live] xfrac.sty depends on latex3

[Karl Berry]

    xfrac.sty from collection-mathextra needs template.sty from
    collection-latex3.  I didn't test any other components of the mh package
    (in which xfrac.sty is included).

    It's up to you to decide whether it is possible to fix that by moving
    around packages or letting mathextra depend on latex3, or whether it
    must be left as an open issue.

Since most mathextra packages don't depend on latex3 (and most likely
never will), I don't want to add that dependency.

But since Morten says all his stuff will "soon" depend on L3, I moved mh
to collection-latex3.

I expect we'll have to revisit this in the future if there end up being
zillions of L3-dependent packages, but for now it seems ok and
expedient.

Best,
Karl