cmd: add a go.local.mod for development

It's essentially a copy of go.mod, but with the gioui.org module replaed
with the parent directory. Useful when wanting to try a change in the
root module as part of a gogio invocation or end-to-end test, such as
either of the following:

	$ cd cmd; go test -modfile=go.local.mod ./...
	$ cd cmd/gogio; go build -modfile=../go.local.mod

Since go.sum can essentially be shared, and since it seems to work with
the go tool, use a symlink. The way the -modfile flag works, if given
foo.mod, it will use the sum file at foo.sum. The only caveat is that
'go mod tidy -modfile=go.mod.local' will try to remove gioui.org lines,
since that module is replaced. So tidy shouldn't be used with -modfile.

In the future, the only upkeep needed for go.local.mod is to update the
external dependencies to mirror changes in go.mod. This is not automated
nor checked by CI, but it could be in the future if it becomes
repetitive or error-prone.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>

.builds/linux.yml

@@ -46,6 +46,7 @@ tasks:
      cd gio/cmd
      go test ./...
      go test -race ./...
+     go test -modfile=go.local.mod ./...
  - test_example: |
      cd gio/example
      go test -race ./...

cmd/go.local.mod

@@ -0,0 +1,14 @@
+module gioui.org/cmd
+
+go 1.13
+
+require (
+	gioui.org v0.0.0-00010101000000-000000000000
+	github.com/chromedp/cdproto v0.0.0-20191114225735-6626966fbae4
+	github.com/chromedp/chromedp v0.5.2
+	golang.org/x/image v0.0.0-20190802002840-cff245a6509b
+	golang.org/x/sync v0.0.0-20190423024810-112230192c58
+	golang.org/x/tools v0.0.0-20190927191325-030b2cf1153e
+)
+
+replace gioui.org => ../

cmd/go.local.sum

@@ -0,0 +1 @@
+go.sum