File inclusion - check for constant defined in the current file

## What problem would the enhancement address for VIP?

The frequency with which "Warning: File inclusion using custom constant (`PREFIX_PATH`). Probably needs manual inspection (WordPressVIPMinimum.Files.IncludingFile.UsingCustomConstant)." shows up just below a line that matches the pattern is high.

Perhaps we can reduce this noise.

## Describe the solution you'd like

See if we can improve the sniff, to see if the constant has been defined in the current file, and if so, skip the violation.

## What code should be reported as a violation?

```php
require PREFIX_PATH_UNDEFINED_IN_THIS_FILE . '/...';
```

## What code should *not* be reported as a violation?

```php
define( 'PREFIX_PATH_DEFINED_IN_THIS_FILE', '...' );
...other code...
require PREFIX_PATH_DEFINED_IN_THIS_FILE . '/...';
```

まず、WordPressVIPMinimum.Files.IncludingFile.UsingCustomConstant を出力する sniff を見つけ、ファイルインクルード式で定数をどのように検出しているかを確認します。未定義の定数の例と、同じファイル内で前に定義された定数についてのカバレッジを追加します。完了条件は、未定義のケースだけが報告されることです。

索引モデルが issue の本文から書いたものです。