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 . '/...';
```

Start by locating the sniff that emits WordPressVIPMinimum.Files.IncludingFile.UsingCustomConstant and inspect how it detects constants in file-inclusion expressions. Add coverage for the undefined constant example and the constant defined earlier in the same file; done means only the undefined case is reported.

Written by the indexing model from the issue text.