Extra extensions directories (#694)

* Allow setting paths for builtin exts and extra dirs

The extra directories aren't used yet, just available from the
environment service and to the shared process.

* Utilize extra builtin extensions path

* Utilize extra extensions directory

* Fix cached mtimes for extra extension dirs

* Simplify extension cache equality check

packages/protocol/src/proto/client_pb.d.ts

@@ -135,6 +135,16 @@ export class WorkingInit extends jspb.Message {
   getExtensionsDirectory(): string;
   setExtensionsDirectory(value: string): void;
 
+  clearExtraExtensionDirectoriesList(): void;
+  getExtraExtensionDirectoriesList(): Array<string>;
+  setExtraExtensionDirectoriesList(value: Array<string>): void;
+  addExtraExtensionDirectories(value: string, index?: number): string;
+
+  clearExtraBuiltinExtensionDirectoriesList(): void;
+  getExtraBuiltinExtensionDirectoriesList(): Array<string>;
+  setExtraBuiltinExtensionDirectoriesList(value: Array<string>): void;
+  addExtraBuiltinExtensionDirectories(value: string, index?: number): string;
+
   serializeBinary(): Uint8Array;
   toObject(includeInstance?: boolean): WorkingInit.AsObject;
   static toObject(includeInstance: boolean, msg: WorkingInit): WorkingInit.AsObject;
@@ -155,6 +165,8 @@ export namespace WorkingInit {
     shell: string,
     builtinExtensionsDir: string,
     extensionsDirectory: string,
+    extraExtensionDirectoriesList: Array<string>,
+    extraBuiltinExtensionDirectoriesList: Array<string>,
   }
 
   export enum OperatingSystem {