Hadrian's `topDirectory` is intended to provide an absolute path to the
root of the GHC tree. However, if the tree is reached via a symlink this

One question here is whether the `canonicalizePath` call is expensive
enough to warrant caching. In a quick microbenchmark I observed that
`canonicalizePath "."` takes around 10us per call; this seems
sufficiently low not to worry.

Alternatively, another approach here would have been to rather move the
canonicalization into `m4/fp_find_root.m4`. This would have avoided
repeated canonicalization but sadly path canonicalization is a hard
problem in POSIX shell.

Addresses #22451.

(cherry picked from commit 5efa9ca5)