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/98396 Reviewed-by: Ian Lance Taylor <iant@golang.org>