wire: update Provider.Out to be a slice of provided types, and keep track of the provided concrete type in ProviderSet.providerMap (google/go-cloud#332)

Update Provider.Out to be a slice of provided types, and keep track
of the provided concrete type in ProviderSet.providerMap, to more
clearly model-named struct providers (which provide both the struct
type and a pointer to the struct type).

Fixes google/go-cloud#325.

internal/wire/testdata/StructPointer/foo/foo.go

@@ -22,7 +22,8 @@ import (
 
 func main() {
 	fb := injectFooBar()
-	fmt.Println(fb.Foo, fb.Bar)
+	e := injectEmptyStruct()
+	fmt.Printf("%d %d %v\n", fb.Foo, fb.Bar, e)
 }
 
 type Foo int
@@ -33,6 +34,8 @@ type FooBar struct {
 	Bar Bar
 }
 
+type Empty struct{}
+
 func provideFoo() Foo {
 	return 41
 }

internal/wire/testdata/StructPointer/foo/wire.go

@@ -24,3 +24,8 @@ func injectFooBar() *FooBar {
 	wire.Build(Set)
 	return nil
 }
+
+func injectEmptyStruct() *Empty {
+	wire.Build(Empty{})
+	return nil
+}

internal/wire/testdata/StructPointer/want/program_out.txt

@@ -1 +1 @@
-41 1
+41 1 &{}

internal/wire/testdata/StructPointer/want/wire_gen.go

@@ -16,3 +16,8 @@ func injectFooBar() *FooBar {
 	}
 	return fooBar
 }
+
+func injectEmptyStruct() *Empty {
+	empty := &Empty{}
+	return empty
+}