feat(overlay): expose interface for custom positions

* Exposes the interface that allows developers to build custom position
strategies for overlays.

src/lib/core/core.ts

@@ -37,24 +37,7 @@ export {DomPortalHost} from './portal/dom-portal-host';
 export * from './platform/index';
 
 // Overlay
-export {Overlay, OVERLAY_PROVIDERS} from './overlay/overlay';
-export {OverlayContainer} from './overlay/overlay-container';
-export {FullscreenOverlayContainer} from './overlay/fullscreen-overlay-container';
-export {OverlayRef} from './overlay/overlay-ref';
-export {OverlayState} from './overlay/overlay-state';
-export {
-  ConnectedOverlayDirective,
-  OverlayOrigin,
-  OverlayModule,
-} from './overlay/overlay-directives';
-export * from './overlay/position/global-position-strategy';
-export * from './overlay/position/connected-position-strategy';
-export * from './overlay/position/connected-position';
-export {ScrollDispatcher} from './overlay/scroll/scroll-dispatcher';
-export {ScrollStrategy} from './overlay/scroll/scroll-strategy';
-export {RepositionScrollStrategy} from './overlay/scroll/reposition-scroll-strategy';
-export {CloseScrollStrategy} from './overlay/scroll/close-scroll-strategy';
-export {NoopScrollStrategy} from './overlay/scroll/noop-scroll-strategy';
+export * from './overlay/index';
 
 // Gestures
 export {GestureConfig} from './gestures/gesture-config';

src/lib/core/overlay/index.ts

@@ -0,0 +1,20 @@
+export {Overlay, OVERLAY_PROVIDERS} from './overlay';
+export {OverlayContainer} from './overlay-container';
+export {FullscreenOverlayContainer} from './fullscreen-overlay-container';
+export {OverlayRef} from './overlay-ref';
+export {OverlayState} from './overlay-state';
+export {ConnectedOverlayDirective, OverlayOrigin, OverlayModule} from './overlay-directives';
+export {ScrollDispatcher} from './scroll/scroll-dispatcher';
+
+export * from './position/connected-position';
+
+// Export pre-defined position strategies and interface to build custom ones.
+export {PositionStrategy} from './position/position-strategy';
+export {GlobalPositionStrategy} from './position/global-position-strategy';
+export {ConnectedPositionStrategy} from './position/connected-position-strategy';
+
+// Export pre-defined scroll strategies and interface to build custom ones.
+export {ScrollStrategy} from './scroll/scroll-strategy';
+export {RepositionScrollStrategy} from './scroll/reposition-scroll-strategy';
+export {CloseScrollStrategy} from './scroll/close-scroll-strategy';
+export {NoopScrollStrategy} from './scroll/noop-scroll-strategy';