Makefile: Add extra-clean to help with testing Makefile

The extra-clean target removes the dependent packages that k3d requires. This makes testing
makefile rules that install those packages easier.

This target helps to expose a bug that the next patch fixes.

Makefile

@@ -24,12 +24,17 @@ GOFLAGS   :=
 BINDIR    := $(CURDIR)/bin
 BINARIES  := k3d
 
+
+# Go Package required
+PKG_GOX := github.com/mitchellh/gox
+PKG_GOLANGCI_LINT := github.com/golangci/golangci-lint/cmd/golangci-lint
+
 export GO111MODULE=on
 
 # go source files, ignore vendor directory
 SRC = $(shell find . -type f -name '*.go' -not -path "./*/*")
 
-.PHONY: all build build-cross clean fmt simplify check
+.PHONY: all build build-cross clean fmt simplify check extra-clean
 
 all: clean fmt check build
 
@@ -43,6 +48,10 @@ build-cross:
 clean:
 	@rm -rf $(BINDIR) _dist/
 
+extra-clean: clean
+	go clean -i $(PKG_GOX)
+	go clean -i $(PKG_GOLANGCI_LINT)
+
 fmt:
 	@gofmt -l -w $(SRC)
 
@@ -59,8 +68,8 @@ HAS_GOX := $(shell command -v gox 2> /dev/null)
 HAS_GOLANGCI  := $(shell command -v golangci-lint 2> /dev/null)
 
 ifndef HAS_GOX
-	go get -u github.com/mitchellh/gox
+	go get -u $(PKG_GOX)
 endif
 ifndef HAS_GOLANGCI
-	go get -u github.com/golangci/golangci-lint/cmd/golangci-lint
+	go get -u $(PKG_GOLANGCI_LINT)
 endif