apicompat could not import a dependency in another repo.

[dmitshur]

See output of https://gci.gopherci.io/analysis/1489:

$ install-deps.sh
No (or unsupported) vendoring tool, using go get
...
$ go vet ./...
$ golint ./...
...
$ apicompat -before 745efbe590e2f613c7121a4bb0f3e39075684af3~4 ./... 978ms
go/types error: 745efbe590e2f613c7121a4bb0f3e39075684af3~4:assets/external.go:4:2: could not import github.com/shurcooL/gofontwoff (can't find import: "github.com/shurcooL/gofontwoff")
$ gosimple ./...
$ staticcheck ./...
$ unused ./...
$ unparam ./...
$ unconvert ./...

apicompat had trouble running because it couldn't import the github.com/shurcooL/gofontwoff dependency. All the other tools seemed to work okay.