Tools via mod, update protoc-gen-go, support new protoc

Wanted to update protoc-gen-go to match the protobuf update. Couldn't get
retool to work properly, despite trying several older versions of Go. The
project is also archived, so tool the time to switch to simple, Go modules
based installs of the tools. This is done without actually changing this
library to need modules, support is only flagged in explicitly to install
those. This is used to install the version of protoc-gen-go that matches the
vendored copy, and the latest version of errcheck is installed.

The protoc check script is updated - the versioning scheme has changed, so we
now just check a minimum of protoc 3.

Generated pb files are re-generated.

CONTRIBUTING.md

@@ -34,8 +34,7 @@ Twirp uses github pull requests. Fork, hack away at your changes and submit. Mos
 
 - Add tests that cover your contribution. Overall code coverage should not decrease.
 - Twirp officially supports the last 3 releases of Go.
-- Protobuf version 3.x.x to generate code with the protoc command.
-- For linters and other tools, we use [retool](https://github.com/twitchtv/retool). If `make setup` is not able to install it, you can install it in your path with `go get github.com/twitchtv/retool` and then install tools with `retool build`.
+- Protobuf version 3.x.x or later to generate code with the protoc command.
 
 ### Running tests
 

Makefile

@@ -1,4 +1,4 @@
-PATH := ${PWD}/_tools/bin:${PWD}/bin:${PATH}
+PATH := ${PWD}/bin:${PATH}
 export GO111MODULE=off
 
 all: setup test_all
@@ -7,10 +7,11 @@ all: setup test_all
 
 setup:
 	./check_protoc_version.sh
-	GOPATH="$$PWD/_tools" GOBIN="$$PWD/_tools/bin" go get github.com/twitchtv/retool
-	./_tools/bin/retool build
 
 generate:
+	# Install dependent tools via modules
+	GO111MODULE=on GOBIN="$$PWD/bin" go install -v google.golang.org/protobuf/cmd/[email protected]
+	GO111MODULE=on GOBIN="$$PWD/bin" go install -v github.com/kisielk/[email protected]
 	# Recompile and install generator
 	GOBIN="$$PWD/bin" go install -v ./protoc-gen-twirp
 	# Generate code from go:generate comments
@@ -19,7 +20,7 @@ generate:
 test_all: setup test test_clientcompat
 
 test: generate
-	./_tools/bin/errcheck ./internal/twirptest
+	./bin/errcheck ./internal/twirptest
 	go test -race ./...
 
 test_clientcompat: generate