path-dependencies are slow

[sigurdm]

When attempting to migrate the SDK to a workspace https://dart-review.googlesource.com/c/sdk/+/397164 we have all third-party dependencies as path-dependencies.

It takes quite long to resolve (~2sec).

Preliminary profiling reveals that canonicalizing the paths is taking most of the time.

The lib/src/io.dart canonicalize() procedure is walking through all parent directories, resolving them in turn, and iterating until everything is solved.

We should perhaps consider using File.resolveSymlinks() that asks the OS for resolution. (I think that might also have more "correct" semantics).

We should also consider caching the canonicalized paths.