fix: FileLocator::findQualifiedNameFromPath() behavior

[kenjis]

Description

• With this PR, FileLocator::findQualifiedNameFromPath() returns a FQCN without leading \

FQCN should not start with \.
get_class() of ::class returns FQCN without leading \.
The following code does not work as expected.:

$class = FileLocator::findQualifiedNameFromPath(...);
if ($class === Foo::class) {
    // do something
}

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[MGatner]

In my opinion this is a bug and could be directed at develop.

[kenjis]

However, it is a breaking change and not urgent.
I will backport to 4.4 if there are many opinions that it should be fixed.