Remove unsafeFlags from Package.swift

[PadraigK]

I removed this locally and did some testing — everything seems fine without it.

Fixes #175

[PadraigK]

My formatter ran on the Package.swift so the diff is bigger than it should be. The interesting changes here are:

90d3910

[migueldeicaza]

Ok, I am a little nervous because I do not recall the reason for this dynamic_lookup change.

So what I am going to do is commit the change, but leave the original stuff commented out, so if we find some odd breakage, we know what to fix.

If there is no breakages in a couple of week, we wipe it out.

[migueldeicaza]

Ok, applied the patch manually as a comment instead, and updated the docs.

[PadraigK]

Thanks!