Rename to go-mod-vendor

[kvedurmu]

In order to implement the Go re-architecture RFC we should rename this buildpack to go-mod-vendor. ID should also be changed to go-mod-vendor.

After this change is made and the buildpack is packit re-written, ordering in the language family Go Buildpack should be something like:

[[order]]
  [[order.group]]
    id = "go-dist"
  [[order.group]]
    id = "go-mod-vendor"
  [[order.group]]
    id = "go-build"
..........................