go/vcs exposes cmd/go/vcs.go from the `go get` command.
- Exported global variables `Verbose` and `ShowCmd` to replace `buildV` and `buildX` from cmd/go/main.go.
- Moved environment building code (envForDir, mergeEnvLists)from cmd/go/main.go to env.go
- Exported Cmd and its methods: Create, Download, Ping, TagSync, and Tags
- Exported ByCmd and FromDir functions for constructing Cmd
- Exported TagCmd
- Exported RepoRoot
- Exported RepoRootForImport* functions for constructing RepoRoot
R=golang-dev, adg, cmang, bradfitz
CC=golang-dev
https://golang.org/cl/12058054
Chris Manghane