Construct package dependencies using Go

For best results, Make should be told about all the files a given
target depends on. For Go programs, this is the program's main Go
file, and all non-test Go files in directories corresponding to
packages the main Go file depends upon (including transitive
dependencies).

This adds a Make macro using go list to retrieve a package's
dependencies, and constructs the corresponding list of files to use as
target prerequisites.

Signed-off-by: Stephen Kitt <skitt@redhat.com>

Makefile

@@ -66,16 +66,20 @@ bin/protoc:
 	unzip protoc-$(PROTOC_VERSION)-linux-x86_64.zip 'bin/*' 'include/*'
 	rm -f protoc-$(PROTOC_VERSION)-linux-x86_64.zip
 
-bin/%/submariner-gateway: main.go $(shell find pkg -not \( -path 'pkg/globalnet*' -o -path 'pkg/routeagent*' -o -path 'pkg/await_node_ready*' \)) pkg/natdiscovery/proto/natdiscovery.pb.go
+basepkg = github.com/submariner-io/submariner
+pkgdeps = $(shell find $$(go list -json $(1) | jq -r '.Imports[] | select(startswith("$(basepkg)")) | sub("$(basepkg)"; ".")') -name '*.go' -not -name '*_test.go')
+
+# natdiscovery.pb.go must be listed explicitly because it might not exist when Make evaluates pkgdeps
+bin/%/submariner-gateway: main.go $(call pkgdeps,.) pkg/natdiscovery/proto/natdiscovery.pb.go
 	GOARCH=$(call dockertogoarch,$(patsubst bin/linux/%/,%,$(dir $@))) ${SCRIPTS_DIR}/compile.sh $@ .
 
-bin/%/submariner-route-agent: $(shell find pkg/routeagent_driver)
+bin/%/submariner-route-agent: $(call pkgdeps,./pkg/routeagent_driver)
 	GOARCH=$(call dockertogoarch,$(patsubst bin/linux/%/,%,$(dir $@))) ${SCRIPTS_DIR}/compile.sh $@ ./pkg/routeagent_driver
 
-bin/%/submariner-globalnet: $(shell find pkg/globalnet)
+bin/%/submariner-globalnet: $(call pkgdeps,./pkg/globalnet)
 	GOARCH=$(call dockertogoarch,$(patsubst bin/linux/%/,%,$(dir $@))) ${SCRIPTS_DIR}/compile.sh $@ ./pkg/globalnet
 
-bin/%/await-node-ready: $(shell find pkg/await_node_ready)
+bin/%/await-node-ready: $(call pkgdeps,./pkg/await_node_ready)
 	GOARCH=$(call dockertogoarch,$(patsubst bin/linux/%/,%,$(dir $@))) ${SCRIPTS_DIR}/compile.sh $@ ./pkg/await_node_ready
 
 nullstring :=