WebUI Polymer3: Handle paths to same folder in polymer_modulizer.py.

<link rel="import" src="foo.html">
needs to be converted to
import './foo.m.js';

Where the "./" prefix is added, otherwise the following error is thrown at
runtime.

Uncaught TypeError: Failed to resolve module specifier "foo.m.js". Relative
references must start with either "/", "./", or "../".

This was discovered while porting md_select_css.html to Polymer3, which
happens to trigger this case.

Bug: 965770
Change-Id: Id22bcb70a2d1bb4035b33b54dbde87bf0321982c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1681343
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#673414}