The motivation for this CL is to support $GOPATH well.
Since we already have a FileSystem interface, implement a
Plan 9-style name space.  Bind each of the $GOPATH src
directories onto the $GOROOT src/pkg directory: now
everything is laid out exactly like a normal $GOROOT and
needs very little special case code.

The filter files are no longer used (by us), so I think they
can just be deleted.  Similarly, the Mapping code and the
FileSystem interface were two different ways to accomplish
the same end, so delete the Mapping code.

Within the implementation, since FileSystem is defined to be
slash-separated, use package path consistently, leaving
path/filepath only for manipulating operating system paths.

I kept the -path flag, but I think it can be deleted too.

Fixes #2234.
Fixes #3046.

R=gri, r, r, rsc
CC=golang-dev
https://golang.org/cl/5711058