Replace tools.go w/ explicit versions in Makefile (#8)

Replace tools.go with explicit versioning of build-time
tooling in the Makefile (e.g. go run [email protected]). Because this
is a library imported by other projects, my preference is to
keep go.mod narrowly focused on the dependencies of the code itself
and not the ancillary tools used by Elastic to maintain its NOTICE.txt
and license headers.

Co-authored-by: Panos Koutsovasilis <[email protected]>

Author: andrewkroh

Makefile

@@ -1,3 +1,6 @@
+GO_LICENSE_DETECTOR := go run go.elastic.co/[email protected]
+GO_LICENSER := go run github.com/elastic/[email protected]
+
 .PHONY: tidy lint test notice write-license-headers
 
 all: tidy lint test notice write-license-headers
@@ -15,7 +18,7 @@ notice:
 	@echo "Generate NOTICE"
 	go mod tidy
 	go mod download
-	go list -m -json all | go run go.elastic.co/go-licence-detector \
+	go list -m -json all | $(GO_LICENSE_DETECTOR) \
 		-includeIndirect \
 		-rules tools/notice/rules.json \
 		-overrides tools/notice/overrides.json \
@@ -25,7 +28,7 @@ notice:
 
 write-license-headers:
 	@echo "Write license headers"
-	go run github.com/elastic/go-licenser \
+	$(GO_LICENSER) \
 		-ext ".go" \
 		-license ASL2 \
 		-licensor "Elasticsearch B.V." \