The test case is
	go doc rand.Float64
The first package it finds is crypto/rand, which does not have a Float64.
Before this change, cmd/doc would stop there even though math/rand
has the symbol. After this change, we get:

	% go doc rand.Float64
	package rand // import "math/rand"

	func Float64() float64

	    Float64 returns, as a float64, a pseudo-random number in [0.0,1.0) from the
	    default Source.
	%

Another nice consequence is that if a symbol is not found, we might get
a longer list of packages that were examined:

	% go doc rand.Int64
	doc: no symbol Int64 in packages crypto/rand, math/rand
	exit status 1
	%

This change introduces a coroutine to scan the file system so that if
the symbol is not found, the coroutine can deliver another path to try.
(This is darned close to the original motivation for coroutines.)
Paths are delivered on an unbuffered channel so the scanner does
not proceed until candidate paths are needed.

The scanner is attached to a new type, called Dirs, that caches the results
so if we need to scan a second time, we don't walk the file system
again. This is significantly more efficient than the existing code, which
could scan the tree multiple times looking for a package with
the symbol.

Change-Id: I2789505b9992cf04c19376c51ae09af3bc305f7f
Reviewed-on: https://go-review.googlesource.com/14921Reviewed-by: Andrew Gerrand <adg@golang.org>