Symbol: add getAlias function

This function returns the last segment string of the symbol (eg: for the
symbol `Foo\Bar\Baz`, it would return `Baz`). This is important because
the part we need to check for each used symbol is comparing its top
level namespace with the aliases of each imported symbol.

Author: sirbrillig

ImportDetection/Symbol.php

@@ -31,6 +31,10 @@ public function getName(): string {
 		return $this->joinSymbolParts($this->tokens);
 	}
 
+	public function getAlias(): string {
+		return $this->tokens[count($this->tokens) - 1]['content'];
+	}
+
 	public function isAbsoluteNamespace(): bool {
 		$type = $this->tokens[0]['type'] ?? '';
 		return $type === 'T_NS_SEPARATOR';