fix(aio): do not fallback to index.html for file requests

[gkalpak]

Previously, all URLs were rewritten to index.html in order to support
deep-linking. This works when navigating to URLs that correspond to existing
resources. E.g. navigating to /tutorial returns index.html and then the
DocViewer takes over and requests tutorial.json.
Navigating to a non-existent URL (e.g. /foo), will return index.html, which
in turn requests (the non-existent) foo.json and throws an error when trying
to parse the returned index.html as JSON.

This commit fixes it by only rewriting URLs that do not request a file (i.e. do
not include a . in the last path segment).

Fixes #15398

[mary-poppins]

The angular.io preview for c4a4008 is available here.

[petebacondarwin]

One thing this doesn't fix is if we tried to navigation to a url that maps to a non-valid JSON file.
There shouldn't be any of those in the system but you never know!
I thought that this was handled client app code, since we have a .catch() RxJS handler in the DocumentService. But it appeared that @IgorMinar was able to make it appear in the staging app.

[gkalpak]

@petebacondarwin, can you give an example URL?

[petebacondarwin]

We don't have any bad ones any more because we blocked the private imports and that VERSION const. So no.

[gkalpak]

So, the issue you mentioned above affects anchor elements pointing to invalid URLs?

UPDATE: I tried that and everything seems to work as expcted 😕
Can you describe a scenario that would not behave as expected.

[petebacondarwin]

I think we do have it working but it was breaking when @IgorMinar was clicking on invalid links in the staging website. But we cannot reproduce that now, since this PR stops the staging site from sending invalid JSON to 404 links.

[petebacondarwin]

I am happy to move on until someone else complains with a repro.

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