goose: allow non-injector code to live along with injectors

Previously, goose would ignore declarations in the //+build gooseinject
files that were not injectors. This meant that if you wanted to write
application-specific providers, you would need to place them in a
separate file, away from the goose injectors. This means that a typical
application would have three handwritten files: one for the abstract
business logic, one for the platform-specific providers, one for the
platform-specific injector declarations.

This change allows the two platform-specific files to be merged into
one: the //+build gooseinject file. goose will now copy these
declarations out to goose_gen.go. This requires a bit of hackery, since
the generated file may have different identifiers for the imported
packages, so goose will do some light AST rewriting to address these
cases.

(Historical note: this was the first change made externally, so also in
here are the copyright headers and other housekeeping changes.)

Reviewed-by: Tuo Shan <shantuo@google.com>
Reviewed-by: kokoro <noreply+kokoro@google.com>

README.md

@@ -72,7 +72,7 @@ package foobarbaz
 
 import (
 	// ...
-	"codename/goose"
+	"github.com/google/go-cloud/goose"
 )
 
 // ...
@@ -116,7 +116,7 @@ package main
 import (
 	"context"
 
-	"codename/goose"
+	"github.com/google/go-cloud/goose"
 	"example.com/foobarbaz"
 )
 
@@ -130,6 +130,9 @@ providers) and can return errors. Arguments to `goose.Use` are the same as
 `goose.NewSet`: they form a provider set. This is the provider set that gets
 used during code generation for that injector.
 
+Any non-injector declarations found in a file with injectors will be copied into
+the generated file.
+
 You can generate the injector by invoking goose in the package directory:
 
 ```