Create in-deptree metadata file

[sdboyer]

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.

[fabulous-gopher]

This issue was moved to golang/dep#437