Commit vendor directory

We need particular versions of our dependencies to build tests and to
build protoc-gen-twirp. We should commit the vendor directory to the
repo to make this simple.

.gitignore

@@ -5,5 +5,3 @@ clientcompat/pycompat/ENV
 *.pyc
 
 build
-
-vendor

Gopkg.toml

@@ -1,11 +1,11 @@
 # The importable parts of Twirp have no external dependencies. They just use the
-# standard library. But Twirp's tests *do* have some dependencies, and this
-# manifest lists them.
+# standard library. But both protoc-gen-twirp and Twirp's tests *do* have some
+# dependencies, and this manifest lists them.
 #
 # All dependencies are specified as overrides, not constraints, so that dep
 # doesn't get confused if someone imports Twirp. We don't want to falsely
 # require any particular versions of these libraries - they are *only* for
-# tests.
+# tests and building main packages.
 
 [[override]]
   name = "github.com/gogo/protobuf"
@@ -22,3 +22,8 @@
 [[override]]
   name = "github.com/stretchr/testify"
   version = "1.2.0"
+
+[prune]
+  unused-packages = true
+  go-tests = true
+  non-go = true