Make an invalid package name a publish error, not warning

[legoktm]

Since invalid names will prevent publishing to packagist, they should
be considered publish errors. If people do not plan on submitting their
package to packagist, they can use the --no-check-publish flag to turn
it into a normal warning again.

[alcohol]

Packagist already fails on warnings, so there is no need for this. The warning also explicitly states they will not be able to publish to packagist.

The $publishErrors array only seems to be used when parsing the composer.json (which either end up in $publishErrors or just $errors). Not sure if it should contain warnings such as these.

[legoktm]

My usecase here is to have CI fail when someone tries creating a package with an uppercase name in it. We currently use composer validate to do that. But since it's just a warning, it doesn't return a failure exit code, causing CI to pass.

This is kind of a follow up to #3804, which officially differentiated between validation errors and publish errors, and let people ignore publish ones.

[alcohol]

Upon further review I agree, in the context of the validate command this should be a publish error. See also ValidateCommand.php#L82-L86.

[Seldaek]

Makes sense yes thanks