Workaround bug in slevomat/coding-standard TypeNameMatchesFileName

For each root namespace, the slevomat rule considers the left-most
match of the given directory in the absolute path of the file. That is,
for /home/user/src/phpstan-src/ the root namespace PHPStan is not
assigned to /home/user/src/phpstan-src/src, but to /home/user/src, which
is obviously wrong.

The bug is known as slevomat/coding-standard#1249 for a long time, but
yet to be fixed. To avoid issues for developers of PHPStan, we can set
a basepath of "." in the PHP CodeSniffer config, which causes paths to
be evaluated relative to the current directory, avoiding
false-positives in the path leading up to the phpstan-src directory.

phpcs.xml

@@ -1,6 +1,7 @@
 <?xml version="1.0"?>
 <ruleset name="PHPStan">
 	<config name="php_version" value="70400"/>
+	<arg name="basepath" value="."/>
 	<arg name="colors"/>
 	<arg name="extensions" value="php"/>
 	<arg name="encoding" value="utf-8"/>