cmd/go: 'go mod download' of a replaced module downloads the go.mod for its replacement

[bcmills]

go mod download of a specific version of a module should download only that module.

Today, if that version happens to have a replacement, go mod download also downloads the go.mod file (but not the .zip file) for its replacement.

$ go mod init golang.org/issue/scratch
go: creating new go.mod: module golang.org/issue/scratch

$ go mod edit -require golang.org/x/[email protected]
$ go mod edit -replace golang.org/x/[email protected]=golang.org/x/[email protected]
$ go mod download -json golang.org/x/[email protected]
go: finding golang.org/x/text v0.3.0
go: finding golang.org/x/text v0.2.0
{
        "Path": "golang.org/x/text",
        "Version": "v0.2.0",
        "Info": "/tmp/tmp.3uwW75Tyk7/_gopath/pkg/mod/cache/download/golang.org/x/text/@v/v0.2.0.info",
        "GoMod": "/tmp/tmp.3uwW75Tyk7/_gopath/pkg/mod/cache/download/golang.org/x/text/@v/v0.2.0.mod",
        "Zip": "/tmp/tmp.3uwW75Tyk7/_gopath/pkg/mod/cache/download/golang.org/x/text/@v/v0.2.0.zip",
        "Dir": "/tmp/tmp.3uwW75Tyk7/_gopath/pkg/mod/golang.org/x/[email protected]",
        "Sum": "h1:WtDSLEtcB5GqbjSlyn8XcYtxjw+SgFMc2RILOvq7CuE=",
        "GoModSum": "h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ="
}

$ find $GOPATH -name '*.mod'
/tmp/tmp.3uwW75Tyk7/_gopath/pkg/mod/cache/download/golang.org/x/text/@v/v0.2.0.mod
/tmp/tmp.3uwW75Tyk7/_gopath/pkg/mod/cache/download/golang.org/x/text/@v/v0.3.0.mod

$ find $GOPATH -name '*.zip'
/tmp/tmp.3uwW75Tyk7/_gopath/pkg/mod/cache/download/golang.org/x/text/@v/v0.2.0.zip

[bcmills]

(Found while investigating #27868.)

[oiooj]

I fixed this problem in my local repo, but from the test script:

go mod download will not follow replacements for explicit module queries.

Maybe we need to think about it again.