go: make go-cross fill main module versions

See mvdan/gofumpt#253.

.bin/go-cross

@@ -1,31 +1,46 @@
 #!/bin/bash
 
-# go-cross [go binary] [version]
+# go-cross [go binary]
 #
 # go-cross builds the main package under the current directory for all the first
-# class Go ports. The optional parameters configure which Go command to build
-# with, and a version override.
+# class Go ports. The optional parameter configures which Go command to build with.
+# The build happens via GOPROXY, which allows embedding a useful module version.
+# We can avoid that step once https://go.dev/issue/50603 is implemented.
 #
-# This script is meant to be simple. If you need more, copy and adapt it.
+# This script is meant to meet my needs. If you need more, copy and adapt it.
 
-# Consistently turn cgo off, even for our local linux/amd64.
+# Consistently turn cgo off, even for our local GOOS/GOARCH.
 export CGO_ENABLED=0
 
 # Since we use GOAMD64=v3 locally,
 # turn that off for cross-builds for greater compatibility.
 export GOAMD64=v1
 
 gocmd=${1:-"go"}
-version=${2:-"$(git describe --always --dirty --tags)"}
-dir=$(basename "$PWD")
+version=${2:-"$(git describe --dirty --tags)"}
+outdir=$PWD
+name=$(basename "$PWD")
+goexe=$($gocmd env GOEXE)
 
-echo "Building version ${version} with $($gocmd version)"
+tmpdir=$(mktemp -d --suffix 'go-cross')
+finish() { rm -rf tmpdir; }
+trap finish EXIT
+
+echo "Building version ${version} with $($gocmd version) from ${tmpdir}"
+
+read modpath pkgpath < <($gocmd list -f '{{.Module.Path}} {{.ImportPath}}' .)
+vcshead=$(git rev-parse HEAD)
+
+# Use a temporary module to resolve via GOPROXY. See the docs above.
+cd "$tmpdir"
+$gocmd mod init test
+$gocmd get ${modpath}@${vcshead}
 
 while read goos goarch; do
 	(
 		export GOOS=$goos GOARCH=$goarch
-		out="${dir}_${version}_${goos}_${goarch}$($gocmd env GOEXE)"
-		$gocmd build -trimpath -ldflags="-w -s -X 'main.version=${version}'" -o="${out}"
+		out=${name}_${version}_${goos}_${goarch}${goexe}
+		$gocmd build -trimpath -ldflags="-w -s" -o="${outdir}/${out}" ${pkgpath}
 		echo $out
 	) &
 done < <(