Update 04-schema.md to reflect package naming in 2.0 (#8777)

UPGRADE-2.0.md

@@ -4,7 +4,7 @@
 
 - If a packages exists in a higher priority repository, it will now be entirely ignored in lower priority repositories. See [repository priorities](https://getcomposer.org/repoprio) for details.
 - Invalid PSR-0 / PSR-4 class configurations will not autoload anymore in optimized-autoloader mode, as per the warnings introduced in 1.10
-- Package names now must comply to our naming guidelines or Composer will abort, as per the warnings introduced in 1.8.1
+- Package names now must comply to our [naming guidelines](doc/04-schema.md#name) or Composer will abort, as per the warnings introduced in 1.8.1
 - Deprecated --no-suggest flag as it is not needed anymore
 - `update` now lists changes to the lock file first, and then the changes applied when installing the lock file to the vendor dir
 - `HTTPS_PROXY_REQUEST_FULLURI` if not specified will now default to false as this seems to work better in most environments

doc/04-schema.md

@@ -34,12 +34,12 @@ separated by `/`. Examples:
 * monolog/monolog
 * igorw/event-source
 
-The name can contain any character, including white spaces, and it's case
-insensitive (`foo/bar` and `Foo/Bar` are considered the same package). In order
-to simplify its installation, it's recommended to define a short and lowercase
-name that doesn't include non-alphanumeric characters or white spaces.
+The name must be lowercased and consist of words separated by `-`, `.` or `_`.
+The complete name should match `^[a-z0-9]([_.-]?[a-z0-9]+)*/[a-z0-9](([_.]?|-{0,2})[a-z0-9]+)*$`.
 
-Required for published packages (libraries).
+The `name` property is required for published packages (libraries).
+
+> **Note:** Before Composer version 2.0, a name could contain any character, including white spaces.
 
 ### description