bug: Check for relative import before looking up context.filename

[vivster7]

If a --format argument is passed to ReplaceImport, we try and represent it as an Import object.

At this point, we won't have a context.filename and will raise an error when trying to normalize a relative import. We don't need to worry about relative imports for imports being passed as --format.

This PR moves the relative import check up first so it'll early exit and skip the context.filename lookup.