Fix previewer sometimes not finding deps file

[maxkatz6]

Fixes #14543

[jp2masa]

I'm curious about this fix. When is the .deps.json file name different from the assembly file name?

I checked workgroupengineering's repro and didn't understand...

[maxkatz6]

@jp2masa when previewer compiles XAML targeting non-entry assembly, but assembly where previewing XAML file is.

Although, while by itself it sounds like a valid use case, it shouldn't really happen with current previewer infrastructure. Especially since we require assembly with BuildAvaloniaApp startup method. And it actually works in 11.0.8 too, but XAML is still compiled with another assembly.
It's something to investigate later though, for now I am keeping this hack, as it can be considered so.

There only were two another PRs related to the previewer #14287 and #14301, each fixing other issues.