Fix: phpdoc cache collision with identical aliases & variable names

[rainbow-alex]

/** @var Foo $x */ in the global scope was cached once for all files, but it can have different semantics depending on the imports used in each file.

I can't provide a playground link because the bug requires multiple files. Please see the test coverage I added for an example.

[ondrejmirtes]

The build failures might be solvable by bumping the cache version number here: https://github.com/phpstan/phpstan-src/pull/471/files#diff-34511260e902425b48cdc48e7434a8768e633d194e55a1273569d20b8310efceR219

[ondrejmirtes]

This line needs to change: $cacheKey = sprintf('%s-phpdocstring-v7-generic-traits', $fileName);

[ondrejmirtes]

Thank you!