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. Fix go-get to use go install mod@latest, if available (Go >= 1.16),
   and fall back to go get -u (deprecated since Go 1.16). The fallback
   can later be removed.

7. Do not check if go-md2man is available, always installing it. This
   fixes the issue of using old version of installed go-md2man, and
   removes the need to have GOBIN make variable.

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

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

Makefile

@@ -1,38 +1,13 @@
-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}
+	go install ${1}@latest || env GO111MODULE=off $(GO) get -u ${1}
 endef
 
 define go-build
@@ -90,9 +65,9 @@ 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 +81,7 @@ build/golangci-lint:
 
 
 .install.md2man:
-	if [ ! -x "$(GOBIN)/go-md2man" ]; then \
-		   $(call go-get,github.com/cpuguy83/go-md2man); \
-	fi
+	$(call go-get,github.com/cpuguy83/go-md2man)
 
 .PHONY: install
 install: