protoc-gen-go: revert some API changes (#577)

Introducing the GoImportPath and GoPackageName types to distingish between
these values changed the exported API of the generator package in a few
small ways. While the API of this package comes with no stability
guarantee, the benefit to the exported part of the change is small.
Revert the exported changes to avoid gratuitous breakage.

Specific changes:
  Generator.ImportPrefix reverts to type string
  Generator.ImportMap reverts to type map[string]string

(There are other externally-visible changes to the package API which
are worth making and will not be reverted, most notably the removal of the
PackageName method from FileDescriptor.)

protoc-gen-go/generator/generator.go

@@ -562,10 +562,10 @@ type Generator struct {
 	Request  *plugin.CodeGeneratorRequest  // The input.
 	Response *plugin.CodeGeneratorResponse // The output.
 
-	Param             map[string]string       // Command-line parameters.
-	PackageImportPath string                  // Go import path of the package we're generating code for
-	ImportPrefix      GoImportPath            // String to prefix to imported package file names.
-	ImportMap         map[string]GoImportPath // Mapping from .proto file name to import path
+	Param             map[string]string // Command-line parameters.
+	PackageImportPath string            // Go import path of the package we're generating code for
+	ImportPrefix      string            // String to prefix to imported package file names.
+	ImportMap         map[string]string // Mapping from .proto file name to import path
 
 	Pkg map[string]string // The names under which we import support packages
 
@@ -629,12 +629,12 @@ func (g *Generator) CommandLineParameters(parameter string) {
 		}
 	}
 
-	g.ImportMap = make(map[string]GoImportPath)
+	g.ImportMap = make(map[string]string)
 	pluginList := "none" // Default list of plugin names to enable (empty means all).
 	for k, v := range g.Param {
 		switch k {
 		case "import_prefix":
-			g.ImportPrefix = GoImportPath(v)
+			g.ImportPrefix = v
 		case "import_path":
 			g.PackageImportPath = v
 		case "paths":
@@ -654,7 +654,7 @@ func (g *Generator) CommandLineParameters(parameter string) {
 			}
 		default:
 			if len(k) > 0 && k[0] == 'M' {
-				g.ImportMap[k[1:]] = GoImportPath(v)
+				g.ImportMap[k[1:]] = v
 			}
 		}
 	}
@@ -807,7 +807,7 @@ func (g *Generator) WrapTypes() {
 			proto3:              fileIsProto3(f),
 		}
 		if substitution, ok := g.ImportMap[f.GetName()]; ok {
-			fd.importPath = substitution
+			fd.importPath = GoImportPath(substitution)
 		} else if p, _, _ := fd.goPackageOption(); p != "" {
 			fd.importPath = p
 		} else {
@@ -1340,7 +1340,7 @@ func (g *Generator) generateHeader() {
 	if importPath == "" {
 		g.P("package ", name)
 	} else {
-		g.P("package ", name, " // import ", g.ImportPrefix+importPath)
+		g.P("package ", name, " // import ", GoImportPath(g.ImportPrefix)+importPath)
 	}
 	g.P()
 
@@ -1400,7 +1400,7 @@ func (g *Generator) generateImports() {
 	// We almost always need a proto import.  Rather than computing when we
 	// do, which is tricky when there's a plugin, just import it and
 	// reference it later. The same argument applies to the fmt and math packages.
-	g.P("import "+g.Pkg["proto"]+" ", g.ImportPrefix+"github.com/golang/protobuf/proto")
+	g.P("import "+g.Pkg["proto"]+" ", GoImportPath(g.ImportPrefix)+"github.com/golang/protobuf/proto")
 	g.P("import " + g.Pkg["fmt"] + ` "fmt"`)
 	g.P("import " + g.Pkg["math"] + ` "math"`)
 	var (
@@ -1427,7 +1427,7 @@ func (g *Generator) generateImports() {
 	for i := range importPaths {
 		importPath := GoImportPath(importPaths[i])
 		packageName := g.GoPackageName(importPath)
-		fullPath := g.ImportPrefix + importPath
+		fullPath := GoImportPath(g.ImportPrefix) + importPath
 		// Skip weak imports.
 		if !strongImports[importPath] {
 			g.P("// skipping weak import ", packageName, " ", fullPath)