Prefer remapping the relative `library/` and `compiler/` directories
This is done to avoid leaking the relative paths to the standard library after the overall of filenames in rust-lang/rust#149709.
Noted that the paths were already leaking before, but to a lesser extent since most (but not all) the paths embedded in the distributed `rlib` were absolute.
In general Cargo compiles workspace members with relative paths, so it's better anyway to remap the relative path.
In addition to our tests I have manually confirmed that it also works as expected for the printed diagnostics paths.
cf. https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/remapping.20of.20the.20standard.20library/near/564093571
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