Account for `-Z simulate-remapped-rust-src-base` when resolving remapped paths
Discovered in #97682, `-Z simulate-remapped-rust-src-base` only partially simulated the behavior of `remap-debuginfo = true`. While the flag successfully simulates the remapping when stdlib's `rmeta` file is loaded, the simulated prefix was not accounted for when the remapped path's local path was being discovered. This caused the flag to not fully simulate the behavior of `remap-debuginfo = true`, leading to inconsistent behaviors.
This PR fixes https://github.com/rust-lang/rust/issues/97682 by also accounting for the simulated path.
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