cmd/cover: replace code using optimized golang.org/x/tools/cover

After CL 179377, this change deletes all the prior cmd/cover code
and instead vendors and type aliases code using the significantly
optimized  golang.org/x/tools/cover, which sped up ParseProfiles by
manually parsing profiles instead of a regex. The speed up was:

name         old time/op   new time/op   delta
ParseLine-12 2.43µs ± 2%   0.05µs ± 8%   -97.98% (p=0.000 n=10+9)

name         old speed     new speed       delta
ParseLine-12 42.5MB/s ± 2% 2103.2MB/s ± 7% +4853.14% (p=0.000 n=10+9)

Fixes #32211.

Change-Id: Ie4e8be7502f25eb95fae7a9d8334fc97b045d53f
Reviewed-on: https://go-review.googlesource.com/c/go/+/249759


Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>