x/tools/gopls: support cancellation

[stamblerre]

The internal/memoize package leaves the generator running if a context was canceled. This means that when a user is typing quickly, we can build up a lot of type-checking and analysis runs. We also don't want to return a context.Canceled inside of the generator, to avoid caching that error. I think that we should add support for canceling the generator to avoid this issue.

We should also consider whether or not packages.Load should take a context. On one hand, this is useful when a user is typing an import manually because it stops us from doing multiple packages.Load in rapid succession, but it also means that we might cancel a useful packages.Load and have to recompute it right away.

/cc @ianthehat @heschik

[heschik]

Closed in https://golang.org/cl/206879.