Port remaining Desugar and Applications errors to the new scheme

[ausmarton]

As described in #1589
Tried porting a couple more of the Errors from Desugar. There are a couple more still remaining, but they seemed to take a bit longer to reproduce the errors for a minimal test case.

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[ausmarton]

Tried porting couple more error messages from Desugar, and came across a redundant check for extension methods being done in Parsers. I've removed the check from Parsers now as suggested by @smarter here: gitter.im/lampepfl/dotty?at=5e485ace0c50da598c1236e9
It seems like that has some implications on #6900

[ausmarton]

All Desugar errors ported to the new scheme now.

I've noticed that for some reason, although the checks are successful, this page seems to show that the "continuous-integration/drone/pr" check is still not reported. In fact, the latest build here: https://dotty-ci.epfl.ch/lampepfl/dotty/4482 is successful.

[anatoliykmetyuk]

LGTM, thanks @ausmarton!

[odersky]

I had to back out most of this. It conflicted in many ways with the preceding PR #8318.

@anatoliykmetyuk In the future, let's not merge error message commits in areas that are still in flux, in particular if they have pending PRs.

Also, let's forget about error messages tests for these kinds of things. I see no value for these tests, and they cause a huge amount of friction for everyone who has to change things in the future.

@ausmarton If you want to try to port to error messages based on the new PR, please go ahead. Please keep in mind to keep things short & simple. I think the advice in #1589 is overblown - following it to the letter slows down our work rather than helping it.