wgo save fails when GO15VENDOREXPERIMENT style layout is used along with multi-gopath layout.

[saml]

wgo save errors out when there's more than one go15vendorexperiment style trees under vendor/

For example, given below directories, wgo save fails:

# go15vendorexperiment
vendor/github.com/
vendor/golang.org/

# multi-gopath
vendor/pkg/
vendor/src/

Notes:

• If there's only one go15vendorexperiment tree (vendor/github.com/ only without vendor/golang.org/), wgo save works.
• If go15vendorexperiment trees are empty (vendor/github.com/ and vendor/golang.org/ are both empty), wgo save works.

Environment:

> go version
go version go1.6.2 linux/amd64
>  go env GOPATH
/home/saml/code

Reproducible steps:

# upgrade wgo
> wgo get -u github.com/skelterjohn/wgo

# create a test project foobar
> mkdir $GOPATH/src/foobar
> cd $GOPATH/src/foobar
> wgo init

# Below is important. 
# Looks like bug needs at least two trees under vendor/ to surface.
# One vendor/github.com and another vendor/golang.org
> wgo get github.com/gorilla/websocket
> wgo get golang.org/x/oauth2 
> cd vendor
> ln -s src/github.com github.com # cp -r is good as well
> ln -s src/golang.org golang.org
> cd ..
> wgo save
exit status 2

[bdandy]

As for me, I think that wgo should be completely migrated to go15vendorexperiment path