made the filesystem loader compatible with Twig 2.0

Without adding the exists() method, the code happens to work by chance,
just because the current implementation of Twig exits() method calls
findTemplate().

But we know that it won't be the case anymore as of Twig 2.0.
merged branch fabpot/twig-filesystem-exists (PR #8677)

This PR was submitted for the master branch but it was merged into the 2.2 branch instead (closes #8677).

Discussion
----------

[TwigBundle] made the filesystem loader compatible with Twig 2.0

| Q             | A
| ------------- | ---
| Bug fix?      | yes (for Twig 2.x)
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Without adding the exists() method, the code happens to work by chance,
just because the current implementation of Twig exits() method calls
findTemplate().

But we know that it won't be the case anymore as of Twig 2.0.

Commits
-------

d00548b [TwigBundle] made the filesystem loader compatible with Twig 2.0