Enhancement: Make no-relative-imports rule stricter

[IllusionMH]

At this moment no-relative-imports dissalows .. only at the beginning of the import path (see #508).

Since next release most likely will be major release it is good opportunity to make "breaking" change disallow .. inside path.

This will be easy check and won't require a lot of effort.

Ideally - disallow not only /../ but also /./ inside of path.

Update rule instead of new one - because it will be basically copy-paste with minor changes in check function (that shouldn't conflict with existing) and error message.

[JoshuaKGoldberg]

Sure, if this gets in within the next couple of days let's fast-track it into 6.0. On the other hand, since no-relative-imports is an odd and rarely used rule (I've been recommending against it for some time now), if this gets in as a 6.1 change instead of 6.0.1, that feels fine too.

[IllusionMH]

Implementation is ready, but there are two questions:

1. Is it critical to preserve Imported module path starts... or External module path starts... or it can be unified to Module path starts... (in any case there will be either import ... or require('...') in error message)?
2. Is there a need to support \ as path separator? Previously only first 1-2 characters were checked and there were no difference for path separators.

[JoshuaKGoldberg]

1. That seems fine. It'd be nice to keep it, but if it overly complicates the implementation details, agreed that it's not critical. We can always file a followup issue to re-add it.
2. Yes, unfortunately. Some Windows developers use \ as path separators AFAIK.