cmd/go/internal/work: pass *Action to toolchain methods

This shortens the args lists but also sets up better for
the content-based staleness changes.

While we're here, delete the now-unused Pkgpath method.

Change-Id: Ic60fa03efbc37a7c7fe9758a1cfa5dddef1a4151
Reviewed-on: https://go-review.googlesource.com/68335Reviewed-by: David Crawshaw <crawshaw@golang.org>