Drop library dependency on tools

This drops the library-level dependency on development tools.
The tools are:
     "github.com/gogo/protobuf/protoc-gen-gogoslick"
     "github.com/golang/mock/mockgen"
     "github.com/kisielk/errcheck"
     "github.com/uber/tchannel-go/thrift/thrift-gen"
     "go.uber.org/thriftrw"
     "go.uber.org/tools/update-license"
     "golang.org/x/lint/golint"
     "golang.org/x/tools/cmd/stringer"
     "honnef.co/go/tools/cmd/staticcheck"

related: uber-go/multierr#35

CHANGELOG.md

@@ -54,6 +54,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
   peer.Chooser, or for the private introspection interface.
 - Peer list peer unavailability errors now provide additional context including
   the number of assigned peers and whether fail-fast is enabled.
+- Drop library dependency on development tools.
 ### Fixed
 - Fixed Streaming Protobuf-flavored-JSON nil pointer panic.
 - Log entries for EOF stream messages are now considered successes to avoid

Dockerfile.1.13

@@ -11,7 +11,7 @@ ADD etc/bin/vendor-build.sh /yarpc/etc/bin/
 # Download and cache dependencies in the image so that we're not constantly
 # re-downloading them locally.
 
-ADD tools.go go.mod go.sum /yarpc/
+ADD tools_test.go go.mod go.sum /yarpc/
 RUN go mod download
 
 ADD internal/examples/go.mod /yarpc/internal/examples/

Dockerfile.1.14

@@ -11,7 +11,7 @@ ADD etc/bin/vendor-build.sh /yarpc/etc/bin/
 # Download and cache dependencies in the image so that we're not constantly
 # re-downloading them locally.
 
-ADD tools.go go.mod go.sum /yarpc/
+ADD tools_test.go go.mod go.sum /yarpc/
 RUN go mod download
 
 ADD internal/examples/go.mod /yarpc/internal/examples/