Fix import path behaviour when using full path

This change makes protoc-gen-star conform to the behaviour
of protoc-gen-go in the presence of an import_path
parameter that contains slashes.

See https://github.com/golang/protobuf/blob/v1.4.2/protoc-gen-go/generator/generator.go#L637-L643
for evidence of this behaviour in protoc-gen-go.

Fixes lyft#77

lang/go/package.go

@@ -23,6 +23,10 @@ func (c context) PackageName(node pgs.Node) pgs.Name {
 	// use import_path parameter ONLY if there is no go_package option in the file.
 	if ip := c.p.Str("import_path"); ip != "" &&
 		e.File().Descriptor().GetOptions().GetGoPackage() == "" {
+
+		if i := strings.LastIndex(ip, "/"); i >= 0 {
+			ip = ip[i+1:]
+		}
 		pkg = ip
 	}
 

lang/go/package_test.go

@@ -21,7 +21,7 @@ func TestPackageName(t *testing.T) {
 		{"unnamed", "names_unnamed"},         // use the proto package if no go_package
 		{"import", "bar"},                    // uses the basename if go_package contains a /
 		{"override", "baz"},                  // if go_package contains ;, use everything to the right
-		{"import_path", "_package"},          // import_path param used if no go_package option
+		{"import_path", "buzz"},              // import_path param used if no go_package option. Should use last part of path.
 		{"mapped", "unaffected"},             // M mapped params are ignored for build targets
 		{"import_path_mapped", "go_package"}, // mixed import_path and M parameters should lose to go_package
 		{"transitive_package", "foobar"},     // go_option gets picked up from other files if present

lang/go/testdata/names/import_path/params

@@ -1 +1 @@
-Mnames/import_path/import_path.proto=foobar,import_path=package
+Mnames/import_path/import_path.proto=foobar,import_path=github.com/fizz/buzz