Dep info contains invalid file paths when rustc-macro is used

[Thinkofname]

Noticed it causing issues with cargo as it uses --emit=dep-info,link and then parses the files as part of the fingerprinting process.

It seems every instance of macro generated code (e.g. via serde_derive) creates a rustc-macro\ source\ code entry into this file which when cargo tries to look up it can't find.

Cargo issue: rust-lang/cargo#3094

[alexcrichton]

Added this to the macros 1.1 tracking issue #35900

(I consider this a blocker for stabilization)