Existing tools use the default value of zero; their behaviour is unchanged.
(*Config).ParseFile is used only from tests.
LGTM=crawshaw, rsc, gri
R=crawshaw, gri, rsc
CC=golang-codereviews
https://golang.org/cl/79290044
CL 49530047 made the (over-)simplifying assumption that the
Path of an ad-hoc (Created) package should default to its
Name, i.e. package declaration.
With this change, the Name is still always computed from the
package declaration (by go/types) but the Path may be
specified by the loader.Config. If "", the value of the Name
is used, which is not globally unique.
R=gri, axwalk
CC=golang-codereviews
https://golang.org/cl/55180043
Alan Donovan