No need to build.Import the dep

We've already figured out the root of the dep, so just use that.

devel* should enable vendor/

Unless there is a class Godep _workspace anyway. This may not be
correct, 100% of the time, but it's likely to be correct most of the
time.

`devel-<short sha>` is newer

Consider devel versions newer than released go versions. This may not
always be the case, but is probably the sanest default.

v67: Handle missing deps a little better

Checkout default git branch

When a git repository is in detached HEAD state use `git show remote
origin` to determine the default branch and check it out before
downloading updates.

This is necessary for `godep restore` to work since
golang/go@4220659

Rewrite update to consider new transitive dependencies

godep update golang.org/x/tools/go/vcs

Support devel-<short sha> go versions

Don't need to build the toolchain either

Handle go version tags

Fixes #448

Obey the go tool's definition of how the go versions build tags work for
the purposes of dependency scanning.

Don't use the current go version, but the recorded go version in
Godeps.json.

FIXME: We still copy the files. This is sub optimal, but probably fine
in the 80% case where people are using the version of go they have
specified in the Godeps.json file. Longer term we need to obey the
gofiles / ignore lists from the package list.