internal/lazyregexp: add a lazy Regexp package

This was implemented as part of go/doc, but it's going to be useful in
other packages. In particular, many packages under cmd/go like web and
vcs make somewhat heavy use of global regexes, which add a non-trivial
amount of init work to the cmd/go program.

A lazy wrapper around regexp.Regexp will make it trivial to get rid of
the extra cost with a trivial refactor, so make it possible for other
packages in the repository to make use of it. While naming the package,
give the members better names, such as lazyregexp.New and
lazyregexp.Regexp.

We're also considering adding some form of a lazy API to the public
regexp package, so this internal package will allow us to get some
initial experience across std and cmd.

For #29382.

Change-Id: I30b0e72871d5267c309786f95f4cb15c68b2393d
Reviewed-on: https://go-review.googlesource.com/c/164040


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>