Get rid of Manifest.TestDependencies() entirely

[sdboyer]

Because the constraint decls no longer imply "requirement," having a separate method on Manifest for declaring test deps is utterly pointless. They should all be lumped in together, and #44 can deal with marking anything that comes in through tests as test-only.

[sdboyer]

note to self (ish) - getting rid of this should mean that pcSliceToMap() can probably also go.

[fabulous-gopher]

This issue was moved to golang/dep#429