[package_info_plus_web] Fix version.json lookup

[jpnurmi]

Description

When creating a new Flutter Web app, the generated index.html contains
the following information:

<!--
  If you are serving your web app in a path other than the root, change the
  href value below to reflect the base path you are serving from.

  The path provided below has to start and end with a slash "/" in order for
  it to work correctly.

  Fore more details:
  * https://developer.mozilla.org/en-US/docs/Web/HTML/Element/base
-->
<base href="/">

The MDN web docs mentioned "document.baseURI", so this patch is using
window.document.baseUri to get the base URL for version.json.

Related Issues

#26

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I updated/added relevant documentation (doc comments with ///).
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Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[deakjahn]

Clever. :-)

[mhadaily]

LGTM