wire/internal/wire: use on-disk GOPATH in generate tests (google/go-cloud#616)

The primary motivation is to permit a move to using go/packages instead
of go/loader. go/packages runs exclusively by shelling out to the go
tool, which precludes use of the in-memory "magic" GOPATH being used
up to this point.

This has a secondary effect of removing a lot of code to support "magic"
GOPATH from the test infrastructure. This is on the whole good, but
necessitated a change in the error scrubbing: since the filenames are
no longer fixed, error scrubbing also must remove the leading
$GOPATH/src lines.

Another related change: since all callers of Generate needed to know the
package path in order to write out wire_gen.go (necessitating a
find-only import search) and Generate already has this information,
Generate now returns this information to the caller. This should further
reduce callers' coupling to Wire's load internals. It also eliminates
code duplication.

This should hopefully shake out any difference in path separators for
running on Windows, but I have not tested that yet.

Updates google/go-cloud#78
Updates google/go-cloud#323

internal/wire/testdata/InterfaceValueDoesntImplement/want/wire_errs.txt

@@ -1 +1 @@
-/wire_gopath/src/example.com/foo/wire.go:x:y: string does not implement io.Reader
+example.com/foo/wire.go:x:y: string does not implement io.Reader