Theme suggestions overriding patterns with hyphens in their ID are not loaded

[gambry]

With #243 we allowed hyphens as pattern id. Although this work perfectly, we haven't fully tested overriding patterns template through theme suggestions.

And indeed if there is a hyphen in the pattern id, overrides are not loaded.

In order to reproduce:

1. Create a pattern with hyphens in is ID, i.e. this-is-a-test
2. In your active theme try to override that pattern, creating pattern-this-is-a-test.html.twig template (of course different from content of pattern template!)
3. Load the pattern in your website, i.e. visiting the Pattern Library section.
4. Override is not loaded.

[gambry]

PR #270 addresses this issue.