cmd/cover: include a package name in the HTML title

A recent change added a title to the HTML coverage report but
neglected to include the package name. Add the package name here.
It's a little trickier than you'd think because there may be multiple
packages and we don't want to parse the files, so we just extract
a directory name from the path of the first file.  This will almost
always be right, and has the advantage that it gives a better result
for package main. There are rare cases it will get wrong, but that
will be no hardship.

If this turns out not to be good enough, we can refine it.

Fixes #38609

Change-Id: I2201f6caef906e0b0258b90d7de518879041fe72
Reviewed-on: https://go-review.googlesource.com/c/go/+/230517


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>