make: Add target for updating dependencies across all modules

To enable more consistent and error-proof dependency management, add a
Make target that will set a dependency version across all submodules
that require it.

Also runs `go mod tidy`. This first ensures the dependency addition is
reverted if the module in question does not require it; it also ensures
that any additional cleanup needed in `go.mod`/`go.sum` is applied.

Makefile

@@ -236,6 +236,19 @@ go-mod-tidy/%:
 	@echo "--> Running go mod tidy ($*)"
 	@cd $* && go mod tidy
 
+.PHONY: go-mod-get
+go-mod-get: $(foreach mod,$(GO_MODULES),go-mod-get/$(mod)) ## Run go get and mod tidy in every module for the given dependency
+
+.PHONY: go-mod-get/%
+go-mod-get/%:
+ifndef DEP_VERSION
+	$(error DEP_VERSION is undefined: set this to <dependency>@<version>, e.g. github.com/hashicorp/go-hclog@v1.5.0)
+endif
+	@echo "--> Running go get ${DEP_VERSION} ($*)"
+	@cd $* && go get $(DEP_VERSION)
+	@echo "--> Running go mod tidy ($*)"
+	@cd $* && go mod tidy
+
 ##@ Checks
 
 .PHONY: fmt