x/vgo: how to list missing modules for imported packages

[dlsniper]

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

go version go1.10.1 windows/amd64 vgo:2018-02-20.1

Does this issue reproduce with the latest release?

Yes.

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

set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\florin\AppData\Local\go-build
set GOEXE=.exe
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOOS=windows
set GOPATH=D:\go
set GOPROXY=
set GORACE=
set GOROOT=C:\Go1.10
set GOTMPDIR=
set GOTOOLDIR=C:\Go1.10\pkg\tool\windows_amd64
set GCCGO=gccgo
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\florin\AppData\Local\Temp\go-build151584538=/tmp/go-build -gno-record-gcc-switches
set VGOMODROOT=

What did you do?

vgo list -m

If possible, provide a recipe for reproducing the error.
https://github.com/dlsniper/vgodemo/tree/9aa4dcefd9a608bf3db6e9cb1b102e508de95b78

What did you expect to see?

I would expect to see that github.com/aws/aws-sdk-go/aws/session or github.com/aws/aws-sdk-go is not a recognized/present module in my project and have a way to add it to go.mod instead.

The only way to get this done currently seems to be running vgo build on the directory, but this means also compiling the application. I would expect to have a way to download missing modules/packages without having to invoke the build command. Maybe I missed it but I can't see anything in help either.

What did you see instead?

Nothing.

Edit:
While it's great that vgo list - m does not resolve packages and its running really fast, so it shouldn't be modified, maybe a standalone command, vgo list - missing, would make it easier to list what packages are missing.

Thank you.

[bcmills]

https://research.swtch.com/vgo-tour says “A side effect of running any vgo command is to update go.mod if necessary,” so it's a bit surprising that vgo list doesn't.

[rsc]

This is almost certainly fixed now. If not now then by CL 121303.

[gopherbot]

Change https://golang.org/cl/121303 mentions this issue: cmd/go/internal/vgo: load packages in parallel