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>

internal/goose/testdata/NamingWorstCaseAllInOne/out.txt

@@ -0,0 +1 @@
+{}