fix: AvaloniaVS can not resolve resource relative path in Previewer

[workgroupengineering]

What does the pull request do?

Fix baseUri

What is the current behavior?

When using xaml like this:

<Application xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="using:ACTest"
             x:Class="ACTest.App">
  <Application.DataTemplates>
    <local:ViewLocator/>
  </Application.DataTemplates>

  <Application.Resources>
    <ResourceDictionary>
      <ResourceDictionary.MergedDictionaries>
        <ResourceInclude Source="Icons.axaml" />
      </ResourceDictionary.MergedDictionaries>

    </ResourceDictionary>

  </Application.Resources>

  <Application.Styles>
    <FluentTheme/>
  </Application.Styles>
</Application>

When opening Designer, the following error is reported

Unable to resolve XAML resource "avares://ACTest/Designer/Icons.axaml" in the "ACTest" assembly. Line 12, position 26.	ACTest	C:\Users\Utente\Source\Repos\ACTest\ACTest\App.axaml	12	

What is the updated/expected behavior with this PR?

No error is raised

How was the solution implemented (if it's not obvious)?

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/Documentation with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

Fixes AvaloniaUI/AvaloniaVS#324

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0038352-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]