user: move userns package to separate module, and retract v0.2.0

commit 86870e7 integrated the userns package
into the github.com/moby/sys/user module, which was included in the v0.2.0
version of the module.

Upon further discussion with maintainers, this may not have been a good
choice; the userns package is related to user-namespaces (uid/gid-mapping),
and while there are some tangential relations with "user", we shouldn't
conflate these concepts by putting both into the same module.

Some downstream projects (containerd, moby, containerd/cgroups) already
accepted patches to switch to the package that's part of the moby/sys/user
module, but none of those patches made it into a release.

This patch:

- moves the userns package to a separate module
- retracts the moby/sys/user v0.2.0 release
- downgrades the minimum go version update for the moby/sys/user module
  to go1.17. Note that CI is no longer testing go1.17, but go1.18 as minimum.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Makefile

@@ -1,4 +1,4 @@
-PACKAGES ?= mountinfo mount sequential signal symlink user
+PACKAGES ?= mountinfo mount sequential signal symlink user userns
 BINDIR ?= _build/bin
 CROSS ?= linux/arm linux/arm64 linux/ppc64le linux/s390x \
 	freebsd/amd64 openbsd/amd64 darwin/amd64 darwin/arm64 windows/amd64
@@ -16,7 +16,7 @@ clean:
 test: test-local
 	set -eu; \
 	for p in $(PACKAGES); do \
-		if $p = user && go version | grep -qv go1.18; then \
+		if $p = userns && go version | grep -qv go1.18; then \
 			(cd $$p; go test $(RUN_VIA_SUDO) -v .); \
 		fi \
 	done
@@ -25,7 +25,7 @@ test: test-local
 tidy:
 	set -eu; \
 	for p in $(PACKAGES); do \
-		if $p = user && go version | grep -qv go1.18; then \
+		if $p = userns && go version | grep -qv go1.18; then \
 			(cd $$p; go mod tidy); \
 		fi \
 	done
@@ -46,7 +46,7 @@ lint: $(BINDIR)/golangci-lint
 	$(BINDIR)/golangci-lint version
 	set -eu; \
 	for p in $(PACKAGES); do \
-		if $p = user && go version | grep -qv go1.18; then \
+		if $p = userns && go version | grep -qv go1.18; then \
 			(cd $$p; \
 			go mod download; \
 			../$(BINDIR)/golangci-lint run); \
@@ -66,6 +66,8 @@ cross:
 		export GOOS=$${osarch%/*} GOARCH=$${osarch#*/}; \
 		echo "# building for $$GOOS/$$GOARCH"; \
 		for p in $(PACKAGES); do \
-			(cd $$p; go build .); \
+			if $p = userns && go version | grep -qv go1.18; then \
+				(cd $$p; go build .); \
+			fi \
 		done; \
 	done

user/go.mod

@@ -1,5 +1,7 @@
 module github.com/moby/sys/user
 
-go 1.21
+go 1.17
 
 require golang.org/x/sys v0.1.0
+
+retract v0.2.0 // Package github.com/moby/sys/user/userns was included in this module, but should've been a separate module; see https://github.com/moby/sys/pull/140#issuecomment-2250644304.

userns/go.mod

@@ -0,0 +1,3 @@
+module github.com/moby/sys/userns
+
+go 1.21