Refactor RootWalker

[quasilyte]

RootWalker does too many things.

• It's used during the indexing (it collects the file meta)
• It's used during the linting (it reports things)
• It contains some code shared with BlockWalker (block walker calls them via RootWalker ptr)
• It's also does some stubs-related things like phpstorm meta namespace handling

Right now it's almost impossible to take it apart without a lot of efforts, but in order to get separated indexing+linting one day, we need to stop making it worse and try to split it into parts that can function independently. Like we did (to some extent) with the BlockWalker.