cmd/covdata: tweak output format for 'go tool covdata percent'
Include some additional whitepace when emitting percentage of
statements covered per package, to make "go tool covdata percent"
output more like "go test -cover" output.
Change-Id: I450cf2bfa05b1eed747cb2f99967314419fa446c
Reviewed-on: https://go-review.googlesource.com/c/go/+/495445 Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Than McIntosh <thanm@google.com>