protoc-gen-go: generate descriptor before plugins (#718)

Avoid interleaving plugin output with the rest of the generated code.
No functional change, but simplifies keeping the v1 and v2 generator
output identical.

Author: neild

protoc-gen-go/generator/generator.go

@@ -1137,12 +1137,11 @@ func (g *Generator) generate(file *FileDescriptor) {
 		g.generateExtension(ext)
 	}
 	g.generateInitFunction()
+	g.generateFileDescriptor(file)
 
 	// Run the plugins before the imports so we know which imports are necessary.
 	g.runPlugins(file)
 
-	g.generateFileDescriptor(file)
-
 	// Generate header and imports last, though they appear first in the output.
 	rem := g.Buffer
 	remAnno := g.annotations