puffin-resolver: add singleton fast path
In many cases, version ranges are actually just pins to a
specific and single version. And we can detect that statically
by examining the range. If we do have a range that is just one
version, then we can ask a `VersionMap` for just that version
instead of iterating over what's in the map until we find one
that satisfies the range.
I had tried this before making `VersionMap` construction lazy,
but it didn't seem to matter much. But helps a lot more now
with a lazy `VersionMap` because it lets us avoid creating a
lot of distributions in memory that we won't ultimately use.
