Try to use is_file() instead of file_exists() in DrupalKernel::findSitePath()

Problem/Motivation

See discussion in #655190: Allow hook_install_tasks() to be called from a profile's .install file (or document that it can't be) for background.

Also see #752730: Remove file_exists() during bootstrap which is trying to remove some unnecessary file_exists() + require_once() from bootstrap to replace them with just include_once() - that would remove most file_exists() in the critical path altogether.

Steps to reproduce

Proposed resolution

Use is_file in DrupalKernel.php

Remaining tasks

Review

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet