There is already a workaround in `compiletest` to deal with custom
`CARGO_HOME` using `-Zignore-directory-in-diagnostics-source-blocks={}`.
A similar need exists when dependencies come from the local `vendor`
directory, which distro builds often use, so now we ignore that too.
Also, `issue-21763.rs` was normalizing `hashbrown-` paths, presumably
expecting a version suffix, but the vendored path doesn't include the
version. Now that matches `[\\/]hashbrown` instead.
f7b2e37f72