use ./... alias instead of PACKAGE_NAMES (#949)

Now that we've stopped our tools from conflicting over `func main`, we
can use the Go tooling's `./...` alias to make them operate over all of
our packages. See `go help packages` for more on `...`. (Those conflicts
were solved in #947).

This means we no longer have to remember to update this variable every
time we want to test or lint a new package in CI and locally.

Updates AUT-192

Makefile

@@ -1,7 +1,6 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
-PACKAGE_NAMES := github.com/mozilla-services/autograph github.com/mozilla-services/autograph/database github.com/mozilla-services/autograph/formats github.com/mozilla-services/autograph/signer github.com/mozilla-services/autograph/signer/apk2 github.com/mozilla-services/autograph/signer/contentsignature github.com/mozilla-services/autograph/signer/contentsignaturepki github.com/mozilla-services/autograph/signer/genericrsa github.com/mozilla-services/autograph/signer/gpg2 github.com/mozilla-services/autograph/signer/mar github.com/mozilla-services/autograph/signer/xpi github.com/mozilla-services/autograph/verifier/contentsignature github.com/mozilla-services/autograph/tools/autograph-monitor
 
 # The GOPATH isn't always on the path.
 GOPATH := $(shell go env GOPATH)
@@ -42,13 +41,13 @@ lint:
 	golangci-lint run
 
 vet:
-	go vet $(PACKAGE_NAMES)
+	go vet ./...
 
 fmt-diff:
 	gofmt -d *.go database/ signer/ tools/autograph-client/ tools/autograph-monitor tools/softhsm/ tools/hawk-token-maker/ tools/make-hsm-ee/ tools/makecsr/ tools/genpki/
 
 fmt-fix:
-	go fmt $(PACKAGE_NAMES)
+	go fmt ./...
 	gofmt -w tools/autograph-client/ tools/softhsm/ tools/hawk-token-maker/ tools/make-hsm-ee/ tools/makecsr/ tools/genpki/
 
 benchmarkxpi:
@@ -58,10 +57,10 @@ showbenchmarkxpi:
 	go tool pprof -web cpu.out
 
 race:
-	go test -race -covermode=atomic -count=1 $(PACKAGE_NAMES)
+	go test -race -covermode=atomic -count=1 ./...
 
 test:
-	go test -v -coverprofile coverage.out -covermode=count -count=1 $(PACKAGE_NAMES)
+	go test -v -coverprofile coverage.out -covermode=count -count=1 ./...
 
 test-in-docker:
 	$(SHELL) -c " \