Make an invalid package name a publish error, not warning

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.

src/Composer/Util/ConfigValidator.php

@@ -102,7 +102,7 @@ class ConfigValidator
             $suggestName = preg_replace('{(?:([a-z])([A-Z])|([A-Z])([A-Z][a-z]))}', '\\1\\3-\\2\\4', $manifest['name']);
             $suggestName = strtolower($suggestName);
 
-            $warnings[] = sprintf(
+            $publishErrors[] = sprintf(
                 'Name "%s" does not match the best practice (e.g. lower-cased/with-dashes). We suggest using "%s" instead. As such you will not be able to submit it to Packagist.',
                 $manifest['name'],
                 $suggestName