Split DataSource & DataSourceView

Summary:
This diff is primarily cosmetic, just pushing code around to make the API more intuitive. Most importantly, DataSource was split into DataSource and DataSourceView classes, the latter being accessible through `datasource.view`.

The benefit of this is two fold:
1. Conceptually it is much clearer now which operations operate on the _source_ records, and which ones on the derived _view_.
2. This will make it easier in the future to support multiple views to be based on a single data source.

This refactoring also nicely found 2 cases where datasource logic and view logic were mixed.

The only semantic change in this diff is that both DataSource and DataSourceView are now iterable, so that one can do a `for (const record of ds)` / `for (const record of ds.view)`

Reviewed By: nikoant

Differential Revision: D26976838

fbshipit-source-id: 3726e92b3c6ee3417dc66cbbe6e288797eecf70e

desktop/flipper-plugin/src/ui/datatable/DataTableManager.tsx

@@ -342,7 +342,7 @@ export function getSelectedItem<T>(
 ): T | undefined {
   return selection.current < 0
     ? undefined
-    : dataSource.getItem(selection.current);
+    : dataSource.view.get(selection.current);
 }
 
 export function getSelectedItems<T>(
@@ -351,7 +351,7 @@ export function getSelectedItems<T>(
 ): T[] {
   return [...selection.items]
     .sort()
-    .map((i) => dataSource.getItem(i))
+    .map((i) => dataSource.view.get(i))
     .filter(Boolean) as any[];
 }