Allow import of dependencies from govendor

[radeksimko]

This will allow import of existing dependencies tracked via govendor.

Transitive dependencies

One caveat I bumped into while testing this out with hashicorp/terraform is that govendor (as opposed to glide) allows transitive dependencies to be pinned to different versions to what the original 1st level dependency requires.

I couldn't find any real benefit of that feature - in fact I think it's better if this treated as a conflict - like it is in Glide.

To solve this in most graceful way I decided to generate initial glide.yaml file which may not be error-less for the first time, but will include all dependencies and 1st glide update will report any conflicts and let the user resolve these, e.g.

[ERROR]	Failed to parse /Users/radek/gopath/src/github.com/hashicorp/terraform/glide.yaml: Import github.com/fsouza/go-dockerclient repeated with different versions 'bf97c77db7c945cbcdbf09d56c6f87a66f54537b' and 'b21e1152923f46354476a3befa5b7aab0f441b4c'

Version vs VersionExact

There are 2 main fields that may define the pinned version. Version is typically more loose (v1 which will match latest v1.*.* tag or branch) and VersionExact is exact match (typically resolved Version). I decided to give priority to VersionExact as I came to the conclusion that user is more likely to benefit from dependency versions that are as close to previous versions as possible.

[sdboyer]

oh oh, it's awesome that you did this! this has been sitting around some TODO list of mine for a while.

i'll have a look at the implementation as soon as i get a chance (i hope later this week). yes, the difference between glide and govendor wrt listing packages vs. projects has been a significant point of discussion for a while; i've also had a couple thoughts on the best way to implement.

[radeksimko]

Assuming the de-facto standard is now go modules (at least for projects that I work on) I'm going to close this PR, but I am happy for anyone else to pick it up, if they still have a need for 3rd-party dependency manager.