Create in-deptree metadata file

[fabulous-gopher]

From @sdboyer on July 19, 2016 23:57

WriteDepTree() should (?) drop a file at the root of all dir trees it writes out place (so, everything explicitly listed in a Lock) that indicates the version actually in use. This isn't foolproof, but is at least a pretty clear indicator of whether that vendor dir was created by a gps-based tool, or not. (We could also go the step further of including checksums of all the files in that tree...thus, basically replicating a vcs. but, uuuuugh)

I'm still undecided on whether this is a good idea. gps needs more exposure to real-world vendor dirs to figure out what to do.

Copied from original issue: sdboyer/gps#69

[sdboyer]

It's not clear whether we'd actually want to do this; if we do, it'd be as related/alternative to #121, as the goal here is ultimately verification.

Closing this out proactively, though, as I don't think it's what we ultimately want.