[DataGrid] Rename missed modules

[DanailH]

Fixes #1260

I skipped ElementSize and InternalGridRowsState as I don't think that they are usable even tho they are public.

[oliviertassinari]

I skipped ElementSize and InternalGridRowsState as I don't think that they are usable even tho they are public.

At some point, could we look into how to not export them to the developers? Maybe it's simple enough to take care of it here? For instance, it seems as simple as:

diff --git a/packages/grid/_modules_/grid/models/index.ts b/packages/grid/_modules_/grid/models/index.ts
index 4d85995e..42cff737 100644
--- a/packages/grid/_modules_/grid/models/index.ts
+++ b/packages/grid/_modules_/grid/models/index.ts
@@ -1,6 +1,5 @@
 export * from './colDef';
 export * from './gridContainerProps';
-export * from './elementSize';
 export * from './gridEditRowModel';
 export * from './gridFeatureMode';
 export * from './gridFilterItem';
 diff --git a/packages/grid/_modules_/grid/components/GridStickyContainer.tsx b/packages/grid/_modules_/grid/components/GridStickyContainer.tsx
index dc1c24a7..8a512aea 100644
--- a/packages/grid/_modules_/grid/components/GridStickyContainer.tsx
+++ b/packages/grid/_modules_/grid/components/GridStickyContainer.tsx
@@ -1,5 +1,5 @@
 import * as React from 'react';
-import { ElementSize } from '../models';
+import { ElementSize } from '../models/elementSize';

[DanailH]

@oliviertassinari I agree, I'll just remove it now and it would be nice to audit again and clean up what is exported and what not.

[oliviertassinari]

For the audit, rebasing #1212 on HEAD might be enough. I did work on #1259 for this very problem.