build.make: support binaries outside of cmd, with optional go.mod

Setting CMDS_DIR in the top-level Makefile before including build.make will
ensure that binaries are picked up from that other directory. This is useful
for lib-volume-populator where an example populator is provided under
"example".

The commands are no longer expected to share the go.mod file with the
project. Entering the command directory and using `go build .` will pick up a
local go.mod if it exists and otherwise fall back to the project's go.mod.

build.make

@@ -21,6 +21,10 @@ SHELL := /bin/bash
 # set in main Makefile of a repository.
 # CMDS=
 
+# Normally, commands are expected in "cmd". That can be changed for a
+# repository to something else by setting CMDS_DIR before including build.make.
+CMDS_DIR ?= cmd
+
 # This is the default. It can be overridden in the main Makefile after
 # including build.make.
 REGISTRY_NAME?=quay.io/k8scsi
@@ -90,15 +94,15 @@ $(CMDS:%=build-%): build-%: check-go-version-go
 		if ! [ $${#os_arch_seen_pre} = $${#os_arch_seen} ]; then \
 			continue; \
 		fi; \
-		if ! (set -x; CGO_ENABLED=0 GOOS="$$os" GOARCH="$$arch" go build $(GOFLAGS_VENDOR) -a -ldflags '$(FULL_LDFLAGS)' -o "./bin/$*$$suffix" ./cmd/$*); then \
+		if ! (set -x; cd ./$(CMDS_DIR)/$* && CGO_ENABLED=0 GOOS="$$os" GOARCH="$$arch" go build $(GOFLAGS_VENDOR) -a -ldflags '$(FULL_LDFLAGS)' -o "$(abspath ./bin)/$*$$suffix" .); then \
 			echo "Building $* for GOOS=$$os GOARCH=$$arch failed, see error(s) above."; \
 			exit 1; \
 		fi; \
 		os_arch_seen+=";$$os-$$arch"; \
 	done
 
 $(CMDS:%=container-%): container-%: build-%
-	docker build -t $*:latest -f $(shell if [ -e ./cmd/$*/Dockerfile ]; then echo ./cmd/$*/Dockerfile; else echo Dockerfile; fi) --label revision=$(REV) .
+	docker build -t $*:latest -f $(shell if [ -e ./$(CMDS_DIR)/$*/Dockerfile ]; then echo ./$(CMDS_DIR)/$*/Dockerfile; else echo Dockerfile; fi) --label revision=$(REV) .
 
 $(CMDS:%=push-%): push-%: container-%
 	set -ex; \
@@ -133,7 +137,7 @@ DOCKER_BUILDX_CREATE_ARGS ?=
 # This target builds a multiarch image for one command using Moby BuildKit builder toolkit.
 # Docker Buildx is included in Docker 19.03.
 #
-# ./cmd/<command>/Dockerfile[.Windows] is used if found, otherwise Dockerfile[.Windows].
+# ./$(CMDS_DIR)/<command>/Dockerfile[.Windows] is used if found, otherwise Dockerfile[.Windows].
 # It is currently optional: if no such file exists, Windows images are not included,
 # even when Windows is listed in BUILD_PLATFORMS. That way, projects can test that
 # Windows binaries can be built before adding a Dockerfile for it.
@@ -146,8 +150,8 @@ $(CMDS:%=push-multiarch-%): push-multiarch-%: check-pull-base-ref build-%
 	export DOCKER_CLI_EXPERIMENTAL=enabled; \
 	docker buildx create $(DOCKER_BUILDX_CREATE_ARGS) --use --name multiarchimage-buildertest; \
 	trap "docker buildx rm multiarchimage-buildertest" EXIT; \
-	dockerfile_linux=$$(if [ -e ./cmd/$*/Dockerfile ]; then echo ./cmd/$*/Dockerfile; else echo Dockerfile; fi); \
-	dockerfile_windows=$$(if [ -e ./cmd/$*/Dockerfile.Windows ]; then echo ./cmd/$*/Dockerfile.Windows; else echo Dockerfile.Windows; fi); \
+	dockerfile_linux=$$(if [ -e ./$(CMDS_DIR)/$*/Dockerfile ]; then echo ./$(CMDS_DIR)/$*/Dockerfile; else echo Dockerfile; fi); \
+	dockerfile_windows=$$(if [ -e ./$(CMDS_DIR)/$*/Dockerfile.Windows ]; then echo ./$(CMDS_DIR)/$*/Dockerfile.Windows; else echo Dockerfile.Windows; fi); \
 	if [ '$(BUILD_PLATFORMS)' ]; then build_platforms='$(BUILD_PLATFORMS)'; else build_platforms="linux amd64"; fi; \
 	if ! [ -f "$$dockerfile_windows" ]; then \
 		build_platforms="$$(echo "$$build_platforms" | sed -e 's/windows *[^ ]* *.exe *[^ ]* *[^ ]*//g' -e 's/; *;/;/g' -e 's/;[ ]*$$//')"; \