go/build: fix ImportDir to report PkgTarget for directories in GOROOT/GOPATH

go/build: fix ImportDir to report PkgTarget for directories in GOROOT/GOPATH

This makes ImportDir("$GOROOT/src/math", 0)
and Import("math", "", 0) equivalent. It was an
oversight that they were not before.

An upcoming change to the go command relies on
the two returning the same results.

Change-Id: I187da4830fae85f8dde673c22836ff2da6801047
Reviewed-on: https://go-review.googlesource.com/75290
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>