The current cmd/doc implementation uses go/build.Import in a few
places to check whether a package is findable and importable.
go/build has limited support for finding packages in modules,
but to do so, build.Import requires knowing the source directory
to use when performing the lookup (so it can find the go.mod file).
Otherwise, it only looks inside the GOPATH workspace.

Start passing the current working directory to build.Import calls,
so that it can correctly look for packages in modules when in cmd/doc
is executed in module mode.

Before this change, cmd/doc in module mode could mistakenly find and
use a package in the GOPATH workspace, instead of the current module.

Since the result of os.Getwd is needed in even more places, assign it
to a local variable in parseArgs now.

Fixes #28992
Updates #26504

Change-Id: I7571618e18420d2d3b3890cc69ade2d97b1962bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/183991
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>