src: move things from internal to vendor for Go 1.7

[bradfitz]

Let's move stuff out of $GOROOT/src/internal and into $GOROOT/src/vendor for Go 1.7.

Related bugs: #13961 and #14031

[bradfitz]

Revert this CL too: https://go-review.googlesource.com/#/c/19384/1

/cc @rsc

[rsc]

To be clear, the only vendor-able stuff in $GOROOT/src/internal is hpack, right?

[bradfitz]

Yes, currently. There was talk of moving singleflight to syncutil too, which has 3-4 copies lately, which are starting to diverge.

[gopherbot]

CL https://golang.org/cl/21063 mentions this issue.

[bradfitz]

Did enough.

[gopherbot]

CL https://golang.org/cl/23114 mentions this issue.