Fix namespace reexports bugs - #2157

This fixes #2156, which actually turned out to expose quite a few separate bugs:

1. src/Chunk.ts:193 needs to trace the full export through reexports, not just the variable trace. This stops a bug where Rollup would render undefined where a module specifier string name should be.
2. Namespace object reexports not working at all in Rollup, which seems to have always been an issue - https://rollupjs.org/repl?version=0.58.2&shareable=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

The fix for (1) is that one-liner, while the fix for (2) is to have namespaces being included populate an import for all reexported modules, and update the linkage graph to ensure this is handled for modules and chunks.

In the process of working on (2) I discovered and resolved two more bugs:

3. Hashes were being cached between different renders, so weren't being calculated correctly for module formats other than the "es" format being rendered first. The fix here is to clear the chunk.renderHash value on each render.

4. Generated imported module names for chunks are not currently being deshadowed - import x from 'chunk2' would not properly deshadow against a local x variable. This fix for this was to change the isExternal check on this deshadowing of module.name to include both externals and other chunk names (src/Chunk.ts:513).