Fix for commands/help relation (dzil-induced).

The issues were filed against `DZ`: rjbs/Dist-Zilla#177 & rjbs/Dist-Zilla#340. Two changes here:
* the hack in *help.pm* (blessing a subref into `GLD`) was not working -- you can easily see this by peeking into `$@` in *commands.pm*; now it does not omit basic usage patter on `dzil help`
* as noted in rjbs/Dist-Zilla#340 it seems reasonable to make `commands` less verbose -- hence the subhead was moved to *help.pm*.

This PR is part of my [advent quest](https://questhub.io/realm/perl/quest/547b4202070ccf750d00010e).

Cheers,
Sergey.