Proposal: add a flag to go get to respect replace directives

[firelizzard18]

What version of Go are you using (go version)?

$ go version
go version go1.16.5 linux/amd64

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOARCH="amd64"
GOOS="linux"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GOVERSION="go1.16.5"
GCCGO="gccgo"

Proposal

go get <url> does not respect replace directives in the top-level go.mod of <url>. #30354 has been referenced by nearly 40 issues in other repositories, many/most of which boil down to "this doesn't build because replace directives aren't respected by go get <url>. Clearly many people expect replace directives in a top level module to be respected when that top-level module is fetched with go get. So I propose to add a flag to go get to enable this behavior.

[seankhliao]

Duplicate of #44840