Improve Makefile: Only install dependencies and rebuild when required

By getting rid of the PHONY install and bundling the go get with its uses we
can prevent unnecessary go gets in `make build` - this speeds up development as
I run `make generate-fixtures` a lot and fetching dependencies takes
comparatively long.

We also add the `-d` flag to go get to get rid of the deprecation warning
```
go get: installing executables with 'go get' in module mode is deprecated.
	To adjust and download dependencies of the current module, use 'go get -d'.
	To install using requirements of the current module, use 'go install'.
	To install ignoring the current module, use 'go install' with a version,
	like 'go install example.com/cmd@latest'.
	For more information, see https://golang.org/doc/go-get-install-deprecation
	or run 'go help get' or 'go help install'.
```

Makefile

@@ -1,16 +1,16 @@
 .PHONY: default
 default: test
 
-.PHONY: install
-install:
-	go get -t ./...
-
-.PHONY: build
-build: install
+go-org: *.go */*.go go.mod go.sum
+	go get -d ./...
 	go build .
 
+.PHONY: build
+build: go-org
+
 .PHONY: test
-test: install
+test:
+	go get -d -t ./...
 	go test ./... -v
 
 .PHONY: setup