cmd/go: go mod why fails for replacement modules

[rogpeppe]

$ go version
go version devel +ce7534ff06 Fri Mar 8 13:46:43 2019 +0000 linux/amd64

When one module is replaced by another, go mod why does not acknowledge the replacement module.

Here's an example (https://godoc.org/github.com/rogpeppe/go-internal/cmd/testscript on the below file to reproduce):

It prints "main module does not need module example.com/c, but example.com/c is definitely in use as the replacement for example.com/a which isn't actually used at all.

# This command succeeds; Note: example.com/d is only
# depended on by example.com/c, but not by example.com/a, which is
# never used.
go mod why -m example.com/d

# This command succeeds, even though the program does not
# depend on example.com/a except as a replacement target.
go mod why -m example.com/a
! stdout 'main module does not need module'

# example.com is definitely used (and it's probably important
# the the user know that it is), but this command fails.
go mod why -m example.com/c
! stdout 'main module does not need module'

-- go.mod --
module m

go 1.11

require (
	example.com/a v1.0.0
	example.com/d v1.0.0 // indirect
)

replace example.com/a => example.com/c v1.0.0
-- go.sum --
example.com/c v1.0.0 h1:+JKa2qCailgQdye6M3nnfrb7q748qDLj2NNTEtN2DJs=
example.com/c v1.0.0/go.mod h1:NeOsx/KTizj35klXP3wYh3O0751aAtYrRoX+a6YAye8=
example.com/d v1.0.0 h1:+FTMPN+4iCiTwx9DTGWb788knewVgy4CGwf1SjyLpXY=
example.com/d v1.0.0/go.mod h1:jpRNKJ+rI4SFCFqRJlfe7G4saIJvHgJss1TcTCWmY18=
-- main.go --
package main

import _ "example.com/a"

func main() {
}
-- .gomodproxy/example.com_a_v1.0.0/.info --
{"Version":"v1.0.0","Time":"2018-10-22T18:45:39Z"}
-- .gomodproxy/example.com_a_v1.0.0/.mod --
module example.com/a
-- .gomodproxy/example.com_a_v1.0.0/a.go --
package a

import _ "example.com/b"
-- .gomodproxy/example.com_a_v1.0.0/go.mod --
module example.com/a

import _ "example.com/b"
-- .gomodproxy/example.com_b_v1.0.0/.info --
{"Version":"v1.0.0","Time":"2018-10-22T18:45:39Z"}
-- .gomodproxy/example.com_b_v1.0.0/.mod --
module example.com/b
-- .gomodproxy/example.com_b_v1.0.0/b.go --
package b
-- .gomodproxy/example.com_b_v1.0.0/go.mod --
module example.com/b
-- .gomodproxy/example.com_c_v1.0.0/.info --
{"Version":"v1.0.0","Time":"2018-10-22T18:45:39Z"}
-- .gomodproxy/example.com_c_v1.0.0/.mod --
module example.com/a
-- .gomodproxy/example.com_c_v1.0.0/a.go --
package a

import _ "example.com/d"
-- .gomodproxy/example.com_c_v1.0.0/go.mod --
module example.com/a

require example.com/d v1.0.0
-- .gomodproxy/example.com_d_v1.0.0/.info --
{"Version":"v1.0.0","Time":"2018-10-22T18:45:39Z"}
-- .gomodproxy/example.com_d_v1.0.0/.mod --
module example.com/d
-- .gomodproxy/example.com_d_v1.0.0/b.go --
package d
import _ "testing"
-- .gomodproxy/example.com_d_v1.0.0/go.mod --
module example.com/d

[bcmills]

We'll revisit this after #26904 is addressed.