docbuilder should report -Wall time of build

[gbaz]

Since we're building anyway, it would be a nice indication to people of the "cost" of any given package.

[hvr]

fwiw, matrix.hho has been recording wall-times (plus a few other metrics provided by GHC) on a per-component granularity in its database and at some point may expose this information via its UI (e.g. as "heat maps" over install plans)