Fix not finding vendor paths, closes psalm#9

Previously paths to dependencies were resolved against the current
working directory and the plugin crashed early if the current working
directory is different with a RuntimeException making Psalm failing
to load the PsalmWordPress\Plugin.

This has been tested against a scenario where psalm is available as
tools/psalm in the project and the composer.json lies within the tools/
folder.

This would also be the case if the vendor directory is not named
"vendor" or when installing psalm with the composer-bin plugin (which is
sometimes suggested in the psalm project [1]).

Fix here is to check for the other packages folders relative to its own
package folder in case the original path relative to the current working
directory is not a directory.

It is done in a way that effectively prevents checking for the name "vendor"
for the [vendor-dir] in case it is installed as a package.

[1]: vimeo/psalm#2905 (comment)
[vendor-dir]: https://getcomposer.org/doc/06-config.md#vendor-dir

Plugin.php

@@ -43,13 +43,57 @@ public function __invoke( RegistrationInterface $registration, ?SimpleXMLElement
 		static::loadStubbedHooks();
 	}
 
+	/**
+	 * resolve a vendor-relative directory-path to the absolute package directory
+	 *
+	 * the plugin must run both from the source file in the repository (current working directory)
+	 * as well as when required as a composer package when the current working directory may not
+	 * have a vendor/ folder and the package directory is detected relative to this file.
+	 *
+	 * @param string $path of a folder, relative, inside vendor/ (composer), must start with 'vendor/' marker
+	 * @return string
+	 */
+	private static function getVendorDir(string $path) : string {
+		$vendor = 'vendor/';
+		$self = 'humanmade/psalm-plugin-wordpress';
+
+		if (0 !== strpos($path, $vendor)) {
+			throw new \BadMethodCallException(
+				sprintf('$path must start with "%s", "%s" given', $vendor, $path)
+			);
+		}
+
+		$cwd = getcwd();
+
+		// prefer path relative to current working directory (original default)
+		$cwdPath = $cwd . '/' . $path;
+		if (is_dir($cwdPath)) {
+			return $cwdPath;
+		}
+
+		// check running as composer package inside a vendor folder
+		$pkgSelfDir = __DIR__;
+		$vendorDir = dirname($pkgSelfDir, 2);
+		if ($pkgSelfDir === $vendorDir . '/' . $self) {
+			// likely plugin is running as composer package, let's try for the path
+			$pkgPath = substr($path, strlen($vendor));
+			$vendorPath = $vendorDir . '/' . $pkgPath;
+			if (is_dir($vendorPath)) {
+				return $vendorPath;
+			}
+		}
+
+		// original default behaviour
+		return $cwdPath;
+	}
+
 	/**
 	 * @return string[]
 	 */
 	private function getStubFiles(): array {
 
 		return [
-			getcwd() . '/vendor/php-stubs/wordpress-stubs/wordpress-stubs.php',
+			self::getVendorDir('vendor/php-stubs/wordpress-stubs') . '/wordpress-stubs.php',
 			__DIR__ . '/stubs/overrides.php',
 		];
 	}
@@ -59,9 +103,11 @@ protected static function loadStubbedHooks() : void {
 			return;
 		}
 
+		$wpHooksDataDir = self::getVendorDir('vendor/johnbillion/wp-hooks/hooks');
+
 		$hooks = array_merge(
-			static::getHooksFromFile( 'vendor/johnbillion/wp-hooks/hooks/actions.json' ),
-			static::getHooksFromFile( 'vendor/johnbillion/wp-hooks/hooks/filters.json' )
+			static::getHooksFromFile( $wpHooksDataDir . '/actions.json' ),
+			static::getHooksFromFile( $wpHooksDataDir . '/filters.json' )
 		);
 
 		static::$hooks = $hooks;
@@ -177,10 +223,10 @@ public static function getFunctionIds() : array {
 	}
 
 	/**
-     * @param  list<PhpParser\Node\Arg>    $call_args
-     *
-     * @return ?array<int, \Psalm\Storage\FunctionLikeParameter>
-     */
+	 * @param  list<PhpParser\Node\Arg>    $call_args
+	 *
+	 * @return ?array<int, \Psalm\Storage\FunctionLikeParameter>
+	 */
 	public static function getFunctionParams(
 		StatementsSource $statements_source,
 		string $function_id,