improve sass partial matching

[FredKSchott]

Changes

• Resolves [plugin-sass] files starting with '_' prefix not getting picked up correctly #1271
• We now match more flexible Sass imports, including those with _ (partials) and folders (foo/_index)
• partials are also never meant to be built directly, so skip them in the final build.
• /cc @heikkilamarko

Testing

• Tests updated.
• Confirmed working in https://github.com/heikkilamarko/mobx-lit-html-app

Docs

• Not needed.

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