Pip packages specified with brackets fail

[timblakely]

gapic-google-cloud-datastore depends on googleapis-common-protos[grpc] and proto-google-cloud-datastore-v1[grpc]. I believe pip canonically drops these extra bracketed tags, but the requirement function attempts to look it up in _requirements by the full key which fails.

[mattmoor]

Does this fix it for you? #18

[mattmoor]

Here's the issue tracking "extras" support: #12 I'm going to resolve in favor of that.

[timblakely]

Thanks, yup, should resolve it.

[mattmoor]

There may be other issues with the google libraries (there is another open issue), but I'm hoping that at least the first-level issue is resolved by my PR :-/