Add a go-package.mk with module support.

MODULE SUPPORT IS HIGHLY EXPERIMENTAL.

This is a version of lang/go/go-package.mk that builds using
Go module support instead of a synthetic GOPATH.

Set GO_MODULE=yes in the package to build using modules, though this will
probably not spark joy right now.

See https://bentsukun.ch/posts/go-mod-support/ for the design.

go/go-package.mk

@@ -0,0 +1,117 @@
+# $NetBSD: go-package.mk,v 1.21 2018/11/30 18:38:19 rillig Exp $
+#
+# This file implements common logic for compiling Go programs in pkgsrc.
+#
+# === Package-settable variables ===
+#
+# GO_MODULE (optional)
+#	Set to "yes" to do a module-based build (EXPERIMENTAL).
+#
+# GO_SRCPATH (required)
+#	The patch that can be used with "go get" to import the current
+#	package. This is usually the URL without the leading protocol.
+#
+#	Examples:
+#		github.com/username/repository
+#		gopkg.in/check.v1
+#
+# GO_DIST_BASE (optional)
+#	Path this package extracts to.
+#
+#	Default:
+#		The last path component of ${GO_SRCPATH}
+#	Examples:
+#		${GITHUB_PROJECT}-${GITHUB_TAG}*
+#		${GITHUB_PROJECT}-${GITHUB_TAG:S/v//}
+#		${DISTNAME}
+#
+# GO_BUILD_PATTERN (optional)
+#	Argument used for 'go install'.
+#	In most cases, the default is fine.
+#
+#	Default:
+#		"${GO_SRCPATH}/...", which means all files below GO_SRCPATH.
+#
+# Keywords: go golang
+#
+
+# Implementation notes
+#
+# The compiled Go code is tied to a specific compiler version, and the
+# compilation is fast. So the plan is:
+#
+# 1. Compile everything.
+# 2. Install binaries into bin/.
+# 3. Install source code and packages into a separate gopkg tree.
+#
+# We implement buildlink by creating a separate tree during the build and
+# linking only the packages explicitly mentioned in dependencies there.
+#
+# All packages build-depend on the default Go release. Go packages should be
+# revbumped when that package is updated.
+#
+
+.include "../../lang/go/version.mk"
+
+_GO_DIST_BASE!=		basename ${GO_SRCPATH}
+GO_DIST_BASE?=		${_GO_DIST_BASE}
+
+.if !empty(${GO_MODULE:M[Yy][Ee][Ss]})
+GO_BUILD_PATTERN?=	all
+.else
+GO_BUILD_PATTERN?=	${GO_SRCPATH}/...
+WRKSRC=			${WRKDIR}/src/${GO_SRCPATH}
+.endif
+
+MAKE_JOBS_SAFE=		no
+INSTALLATION_DIRS+=	bin gopkg
+USE_TOOLS+=		pax
+
+BUILD_DEPENDS+=		${GO_PACKAGE_DEP}
+
+GOTOOLDIR=		go${GOVERSSUFFIX}/pkg/tool/${GO_PLATFORM}
+
+PRINT_PLIST_AWK+=	/^@pkgdir bin$$/ { next; }
+PRINT_PLIST_AWK+=	/^@pkgdir gopkg$$/ { next; }
+
+.if !empty(${GO_MODULE:M[Yy][Ee][Ss]})
+MAKE_ENV+=	GO111MODULE=on GOPROXY=file://${WRKDIR}/.proxy
+.else
+MAKE_ENV+=	GO111MODULE=off GOPATH=${WRKDIR}:${BUILDLINK_DIR}/gopkg 
+.endif
+MAKE_ENV+=	GOCACHE=${WRKDIR}/.cache/go-build
+
+.if !target(post-extract) && empty(${GO_MODULE:M[Yy][Ee][Ss]})
+post-extract:
+	${RUN} ${MKDIR} ${WRKSRC}
+	${RUN} ${RM} -fr ${WRKDIR}/${GO_DIST_BASE}/.hg
+	${RUN} ${MV} ${WRKDIR}/${GO_DIST_BASE}/* ${WRKSRC}
+.endif
+
+.if !target(do-build)
+do-build:
+	${RUN} ${PKGSRC_SETENV} ${MAKE_ENV} ${GO} install -v ${GO_BUILD_PATTERN}
+.endif
+
+.if !target(do-test)
+do-test:
+	${RUN} ${PKGSRC_SETENV} ${TEST_ENV} ${MAKE_ENV} ${GO} test -v ${GO_BUILD_PATTERN}
+.endif
+
+.if !target(do-install)
+do-install:
+	${RUN} cd ${WRKDIR}; [ ! -d bin ] || ${PAX} -rw bin ${DESTDIR}${PREFIX}
+	${RUN} cd ${WRKDIR}; [ ! -d pkg ] || ${PAX} -rw src pkg ${DESTDIR}${PREFIX}/gopkg
+.endif
+
+_VARGROUPS+=		go
+_PKG_VARS.go=		GO_SRCPATH GO_DIST_BASE GO_BUILD_PATTERN
+_USER_VARS.go=		GO_VERSION_DEFAULT
+_SYS_VARS.go=		GO GO_VERSION GOVERSSUFFIX GOARCH GOCHAR \
+			GOOPT GOTOOLDIR GO_PLATFORM
+_DEF_VARS.go=		GO14_VERSION GO19_VERSION GO110_VERSION \
+			GO111_VERSION INSTALLATION_DIRS MAKE_JOBS_SAFE \
+			NOT_FOR_PLATFORM ONLY_FOR_PLATFORM SSP_SUPPORTED \
+			WRKSRC
+_USE_VARS.go=		GO_PACKAGE_DEP
+_SORTED_VARS.go=	INSTALLATION_DIRS *_FOR_PLATFORM