Allow go2idl to generate non-Go file types

Remove some indentation from file generation for specific languages,
allow SnippetWriter's io.Writer to be accessed, and add Path to
types.Name for languages where Package and Path can be disjoint.

cmd/libs/go2idl/parser/parse.go

@@ -83,6 +83,11 @@ func New() *Builder {
 	}
 }
 
+// AddBuildTags adds the specified build tags to the parse context.
+func (b *Builder) AddBuildTags(tags ...string) {
+	b.context.BuildTags = append(b.context.BuildTags, tags...)
+}
+
 // Get package information from the go/build package. Automatically excludes
 // e.g. test files and files for other platforms-- there is quite a bit of
 // logic of that nature in the build package.