As defined in the `go help packages`:
Many commands apply to a set of packages:
go action [packages]
Usually, [packages] is a list of import paths.
This patch introduces automatic lookup of said packages from GOPATH
using `go list`, and provides them as options go subcmds.
The tool subcommand had a "-f" flag to disallow file completions which is wrong: most of the tools there require a file/directory argument.
Since we're here, also limit "go tool compile" to only match Go source files.
