Overhaul filename handling for cross-compiler consistency This PR overhauls the way we handle filenames in the compiler and `rmeta` in order to achieve achieve cross-compiler consistency (ie. having the same path no matter if the filename was created in the current compiler session or is coming from `rmeta`). This is required as some parts of the compiler rely on consistent paths for the soundness of generated code (see rust-lang/rust#148328). In order to achieved consistency multiple steps are being taken by this PR: - by making `RealFileName` immutable - by only having `SourceMap::to_real_filename` create `RealFileName` - currently `RealFileName` can be created from any `Path` and are remapped afterwards, which creates consistency issue - by also making `RealFileName` holds it's working directory, embeddable name and the remapped scopes - this removes the need for a `Session`, to know the current(!) scopes and cwd, which is invalid as they may not be equal to the scopes used when creating the filename In order for `SourceMap::to_real_filename` to know which scopes to apply `FilePathMapping` now takes the current remapping scopes to apply, which makes `FileNameDisplayPreference` and company useless and are removed. This PR is split-up in multiple commits (unfortunately not atomic), but should help review the changes. Unblocks https://github.com/rust-lang/rust/pull/147611 Fixes https://github.com/rust-lang/rust/issues/148328