Invert "does this entry match the filter" logic

[EvanHahn]

To be merged into #940.

Before this change, we turned filters into a list of path prefixes and then checked entry paths against those prefixes. Rough pseudocode:

function doesEntryMatchFilter({ path }, filter) {
  const pathPrefixes = pathPrefixesFromFilter(filter)
  return pathPrefixes.some((prefix) => path.startsWith(prefix))
}

For performance and simplicity, I think it's cleaner if we "look up" entry paths in the existing filter object. Rough pseudocode:

function doesEntryMatchFilter({ path }, filter) {
  const { type, variant } = parsePath(path)
  return filter[type]?.includes(variant)
}

I think this has two advantages:

• It's less code. We don't need to worry about de-duping paths (e.g., /photos/original versus /photos/). We don't need to worry about sketchy paths (e.g., /photos/garbage/../original).
• It's faster (at least, in theory). Rather than having to iterate over every path prefix, we only need to iterate over the variants of each path. (This could be further optimized with a Set.)