cmd/go: 'go list -mod=readonly' erroneously resolves imports from replaced modules

[bcmills]

When -mod=readonly is set, go list should refuse to resolve missing dependencies, and normally does.

However, if the dependency can be satisfied by adding a requirement on a module mentioned in a replace directive, the usual lookup disabled error message is suppressed, and go list instead fails with updates to go.mod needed:

example.com$ go version
go version devel +150bd4ff Wed Sep 23 07:51:17 2020 +0000 linux/amd64

example.com$ go list -mod=readonly example.com/x
cannot find module providing package example.com/x: import lookup disabled by -mod=readonly

example.com$ go mod edit -replace=example.com/[email protected]=./x

example.com$ go list -mod=readonly example.com/x
go: found example.com/x in example.com/x v0.1.0
go: updates to go.mod needed, disabled by -mod=readonly

-- go.mod --
module example.com

go 1.16
-- x/go.mod --
module example.com/x

go 1.16
-- x/x.go --
package x

[gopherbot]

Change https://golang.org/cl/258220 mentions this issue: cmd/go/internal/modload: rework replacements in the Query functions

[gopherbot]

Change https://golang.org/cl/263266 mentions this issue: cmd/go/internal/modload: fix sort condition in (*replacementRepo).Versions

[bcmills]

Fixed by CL 258220.