[Config] Improved FileLocator->isAbsolutePath()

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

The `isAbsolutePath()` method used array notation to access string characters. Passing an empty string to it, would therefore generate an undefined offset error. Moreover, passing an array that had a slash or backslash in its first element would incorrectly pass as well, e.g. `array('/', 'what', 'ever')`.

Using a simple regular expression here seems like the way to go. It is shorter and easier to read.