misc: Also check for .zst-compressed variants of files

Initially I thought of breaking off the Stat + error check call into its own function as to reduce repetition, but given that it's only useful in this situation where it only happens twice anyway, I'm not sure it actually would reduce complexity.

Additionally, this means that .zst-compressed variants of files will be searched for in all contexts where this function is used. I'm not sure this is desirable. Suggestions welcome! I'm not familiar with Go as a language either, so general code style suggestions are welcome as well.

Closes #39 (closed)