In golang.org/cl/59413, the two-argument behavior of cmd/doc was changed
to use findPackage instead of build.Import, meaning that the tool was
more consistent and useful.

However, it introduced a regression:

	$ go doc bytes Foo
	doc: no such package: bytes

This is because the directory list search would not find Foo in bytes,
and reach the end of the directory list - thus resulting in a "no such
package" error, since no directory matched our first argument.

Move the "no such package" error out of parseArgs, so that the "loop
until something is printed" loop can have control over it. In
particular, it is useful to know when we have reached the end of the
list without any exact match, yet we did find one package matching
"bytes":

	$ go doc bytes Foo
	doc: no symbol Foo in package bytes

While at it, make the "no such package" error not be fatal so that we
may test for it. It is important to have the test, as parseArgs may now
return a nil package instead of exiting the entire program, potentially
meaning a nil pointer dereference panic.

Fixes #22810.

Change-Id: I90cc6fd755e2d1675bea6d49a1c13cc18ac9bfb9
Reviewed-on: https://go-review.googlesource.com/78677
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>