Correctly handle symlinks when prefetching.

In https://github.com/bazelbuild/bazel/commit/5c4cf47a131c84506aad9ce0e014c6643c31a4ac, we replaced `path = path.getRelative(path.readSymbolicLink())` with `path = path.resolveSymbolicLink`. While the former one is wrong for relative symlinks, it actually works for absolute symlinks. The latter one, however, doesn't work if the target doesn't exist on local file system -- and this is the case for most of builds using our internal version of BwoB.

This CL fixes that issue by correctly handle both absolute and relative symlinks. Additionally, it correctly handles the case where a symlink points to a tree output.

Moved symlink integration tests to the base test class.

PiperOrigin-RevId: 549921885
Change-Id: If4009525499f4d0625e7725faf4bb0675984c299