Add back build prerequisite go_deps.bzl

Make go_deps.bzl a non-phony prerequisite for build
Only update go_deps.bzl if `go.mod`  was changed, only reset `go_deps.bzl` if it does not exists or is empty
Add realclean target that also deletes `go_deps.bzl` in addition to the rest of the bazel cleanup

Makefile

@@ -1,20 +1,25 @@
 
-.PHONY: all bazel build clean gazelle package package_deb package_rpm
+.PHONY: all bazel build clean realclean package package_deb package_rpm
 
 build: bazel
 #go build -ldflags "-X github.com/netsec-ethz/bootstrapper/config.versionString="$(./.bazel-build-env | awk '{print $2}')
 
-bazel:
+bazel: go_deps.bzl
 	rm -f bin/*
 	bazel build //:bootstrapper
 	cp bazel-bin/bootstrapper bin/
 
-all: go_deps.bzl gazelle build test package
+all: build test package
 
 clean:
 	bazel clean
 	rm -f bin/*
 
+realclean: clean
+	bazel clean --expunge
+	rm ~/.cache/bazel
+	rm -f go_deps.bzl
+
 package: package_deb
 
 package_deb: build
@@ -25,9 +30,6 @@ package_rpm: build
 	bazel build //:scion-bootstrapper-rpm
 	cp bazel-bin/scion-bootstrapper-*.*.rpm bin/
 
-gazelle: go.mod go_deps.bzl
-	bazel run //:gazelle -- update-repos -from_file=go.mod -to_macro=go_deps.bzl%go_deps
-
 define go_deps_boilerplate
 # Generated from go.mod by gazelle. DO NOT EDIT
 load("@bazel_gazelle//:deps.bzl", "go_repository")
@@ -36,8 +38,11 @@ def go_deps():
   pass
 endef
 
-go_deps.bzl:
+go_deps.bzl: go.mod
+ifeq (,$(wildcard go_deps.bzl))
 	$(file > ./go_deps.bzl,$(go_deps_boilerplate))
+endif
+	bazel run //:gazelle -- update-repos -from_file=go.mod -to_macro=go_deps.bzl%go_deps -prune
 
 test: build
 	bazel test --config=unit --test_output=errors ...

README.md

@@ -156,4 +156,4 @@ The latter should return a file containing the requested TRC.
 ## Contributing
 
 When contributing, please run `make all` in order to make sure that all targets build and to run the linter.
-To update the `go_deps.bzl` file and the dependencies Bazel uses from the `go.mod` module file, run `make gazelle`.
+To generate the `go_deps.bzl` file from scratch, delete it and run `make go_deps.bzl`.