Makefile: simplify for modern Go

Quite a few changes, mostly removing old stuff though.

1. "GO111MODULE=off" is no longer required to be set by default (and
   it used to be overridden below anyway).

2. "go build" no longer requires explicit "-mod=vendor", as this is the
   default since go 1.14.

3. GOPROXY is set to proxy.golang.org by default since go 1.13.

4. Always use default GOPATH (which is $HOME/go since Go 1.8; earlier
   releases needed to set it explicitly).

5. Drop the code that handles multiple comma-separated GOPATH elements
   (when using modules, GOPATH is no longer used for resolving imports,
   which basically means using multiple paths is useless).

6. Drop go-get macro (which is used to install md2man), use go install
   directly (supported since Go 1.16). While at it, do not check if
   go-md2man is available, install it unconditionally. This removes
   the need to have GOBIN make variable.

7. Remove GOPKGBASEDIR and GOPKGBASEDIR, defined by Makefile, were never
   used.

8. Rm PROJECT, use relative paths in test-unit target.

Signed-off-by: Kir Kolyshkin <[email protected]>

Makefile

@@ -1,39 +1,9 @@
-export GO111MODULE=off
-
 GO ?= go
 GO_BUILD=$(GO) build
-# Go module support: set `-mod=vendor` to use the vendored sources
-ifeq ($(shell go help mod >/dev/null 2>&1 && echo true), true)
-	GO_BUILD=GO111MODULE=on $(GO) build -mod=vendor
-endif
 BUILDTAGS := containers_image_openpgp,systemd,exclude_graphdriver_devicemapper
 DESTDIR ?=
 PREFIX := /usr/local
 CONFIGDIR := ${PREFIX}/share/containers
-PROJECT := github.com/containers/common
-
-# Enforce the GOPROXY to make sure dependencies are resovled consistently
-# across different environments.
-export GOPROXY := https://proxy.golang.org
-
-# If GOPATH not specified, use one in the local directory
-ifeq ($(GOPATH),)
-export GOPATH := $(CURDIR)/_output
-unexport GOBIN
-endif
-FIRST_GOPATH := $(firstword $(subst :, ,$(GOPATH)))
-GOPKGDIR := $(FIRST_GOPATH)/src/$(PROJECT)
-GOPKGBASEDIR ?= $(shell dirname "$(GOPKGDIR)")
-
-GOBIN := $(shell $(GO) env GOBIN)
-ifeq ($(GOBIN),)
-GOBIN := $(FIRST_GOPATH)/bin
-endif
-
-define go-get
-	env GO111MODULE=off \
-		$(GO) get -u ${1}
-endef
 
 define go-build
 	CGO_ENABLED=0 \
@@ -90,9 +60,9 @@ vendor-in-container:
 
 .PHONY: vendor
 vendor:
-	GO111MODULE=on $(GO) mod tidy
-	GO111MODULE=on $(GO) mod vendor
-	GO111MODULE=on $(GO) mod verify
+	$(GO) mod tidy
+	$(GO) mod vendor
+	$(GO) mod verify
 
 .PHONY: install.tools
 install.tools: build/golangci-lint .install.md2man
@@ -106,9 +76,7 @@ build/golangci-lint:
 
 
 .install.md2man:
-	if [ ! -x "$(GOBIN)/go-md2man" ]; then \
-		   $(call go-get,github.com/cpuguy83/go-md2man); \
-	fi
+	$(GO) install github.com/cpuguy83/go-md2man/v2@latest
 
 .PHONY: install
 install:
@@ -124,8 +92,8 @@ test: test-unit
 test-unit:
 	go test --tags $(BUILDTAGS) -v ./libimage
 	go test --tags $(BUILDTAGS) -v ./libnetwork/...
-	go test --tags $(BUILDTAGS) -v $(PROJECT)/pkg/...
-	go test --tags remote,seccomp,$(BUILDTAGS) -v $(PROJECT)/pkg/...
+	go test --tags $(BUILDTAGS) -v ./pkg/...
+	go test --tags remote,seccomp,$(BUILDTAGS) -v ./pkg/...
 
 .PHONY: codespell
 codespell: