Before, an argument that started ./ or ../ was not treated as
a package relative to the current directory. Thus

	$ cd $GOROOT/src/text
	$ go doc ./template

could find html/template as $GOROOT/src/html/./template
is a valid Go source directory.

Fix this by catching such paths and making them absolute before
processing.

Fixes #23383.

Change-Id: Ic2a92eaa3a6328f728635657f9de72ac3ee82afb
Reviewed-on: https://go-review.googlesource.com/98396Reviewed-by: Ian Lance Taylor <iant@golang.org>