cmd/go: add -modfile and -modcacherw to go tool

When adding support for module tools, we added the ability for `go tool`
to edit the module cache. For users with `GOFLAGS=-modcacherw` this
could have led to a situation where some of the files in the mod cache
were unexpectedly not deletable.

We also allow -modfile so that people can select which module they are
working in when looking for tools.

We still do not support arbitrary build flags for tools with `go tool`.
If you want those, use `go run` or `go build`, etc. instead.

Updates #48429

Change-Id: Ic3c56bb8b6ba46114196465ca6ee2dcb08b9dcc0
Reviewed-on: https://go-review.googlesource.com/c/go/+/632575
Reviewed-by: Michael Matloob <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

src/cmd/go/internal/tool/tool.go

@@ -66,6 +66,7 @@ func isGccgoTool(tool string) bool {
 
 func init() {
 	base.AddChdirFlag(&CmdTool.Flag)
+	base.AddModCommonFlags(&CmdTool.Flag)
 	CmdTool.Flag.BoolVar(&toolN, "n", false, "")
 }
 

src/cmd/go/testdata/script/mod_cache_rw.txt

@@ -42,6 +42,15 @@ cp $WORK/extraneous.txt $GOPATH/pkg/mod/rsc.io/[email protected]/extraneous_file.go
 ! go mod verify
 [!root] ! cp $WORK/extraneous.txt $GOPATH/pkg/mod/rsc.io/[email protected]/go.mod
 
+# Repeat part of the test with 'go tool'
+go mod edit -tool example.com/tools/cmd/hello -require example.com/[email protected]
+go mod download -modcacherw example.com/tools
+go clean -modcache
+go tool -modcacherw hello
+cp $WORK/extraneous.txt $GOPATH/pkg/mod/example.com/[email protected]/extraneous_file.go
+! go mod verify
+[!root] ! cp $WORK/extraneous.txt $GOPATH/pkg/mod/example.com/[email protected]/cmd/hello/hello.go
+
 
 -- $WORK/extraneous.txt --
 module oops

src/cmd/go/testdata/script/modfile_flag.txt

@@ -48,6 +48,10 @@ go build -n -mod=mod .
 go test -n -mod=mod .
 go get rsc.io/quote
 
+# 'go tool' and tool management should work.
+go get -tool example.com/tools/cmd/[email protected]
+grep cmd/hello go.alt.mod
+go tool hello
 
 # 'go mod vendor' should work.
 go mod vendor