vendoring: try to catch more vendoring issues & fixup an existing one

[ijc]

At the moment vendor.conf is missing entries for github.com/tonistiigi/llb-gobuild
and github.com/morikuni/aec due in part because of a combination of
LK4D4/vndr#62 and LK4D4/vndr#63.

The issue vndr#63 (related to lack of github.com/morikuni/aec) can be worked
around by removing the vendor directory before rerunning vndr, so do so.

Due to vndr#62 the issue with github.com/tonistiigi/llb-gobuild cannot be
detected at the moment, but pass -strict to vndr in anticipation of a fix
there.

This will make the CI fail on this PR (I hope). Once it has done so I will push an extra patch which fixes things by adding the github.com/morikuni/aec entry to vendor.conf.

I will not add the github.com/tonistiigi/llb-gobuild since the user examples/gobuild/main.go is marked // +build ignore, with current vndr that produces a warning (harmless due too LK4D4/vndr#62 even we add -strict here) but with newer vndr it does not.

[ijc]

CI failed as desired: https://travis-ci.org/moby/buildkit/builds/350858589#L2293

Will now push the fix.

[tonistiigi]

Yes, not vendoring llb-gobuild was intentional as it is only used for examples. Although I think we should still try to build it in tests.

LGTM