…mented (#310)

The code settings up the command-line switch was passing `inv.Dir` as the
default value, which at the time of the call is always `"."`. This means
that the later code that sets `inv.WorkDir = inv.Dir` won't run because
`inv.WorkDir` is not the empty string.

The command line -help documentation reads:

      -w <string>
            working directory where magefiles will run (default -d value)

But given the above, this is not the observed behavior.

Now `inv.WorkDir` is defaulted to the empty string, and the code works
as expected.

Co-authored-by: Nate Finch <[email protected]>

* target: rework modtime comparison logic

* fix NewestModTime, add tests for NewestModTime, OldestModTime

* added tests for Dir when target is Dir

* updates per #324

* I guess arguments work now
* add mg.F