`depot status` and `depot get` chose a local depot tree with `--target
local`, resolving the root from `DEPOT_DIR`. That split one decision over
two flags and left `--target local` valid with no directory anywhere.
Adopt the nwsync convention instead: `--out DIR` names the tree on disk in
the same breath as choosing it, and `--target` now names remote backends
only. The two are mutually exclusive. `--target local` stays as a
deprecated alias for `--out $DEPOT_DIR` so existing callers keep working,
and `status --source` is now ignored with a warning instead of silently
accepted.
Also split positionals off by hand in `depot get`, so the documented
`depot get <sha> <dest> --out DIR` form parses its flags at all — Go's
flag package stops at the first positional.
Closes#66
archvillainette