policy: normalize local policy paths

[crazy-max]

needs #3943

Was testing some policies on multiple machines and it fails in a case on Windows because fs.FS paths must use forward slashes, but local policy paths were cleaned with filepath.Clean, which produces backslashes on Windows. That made valid policy files fail to load with invalid argument:

mkdir bx-policy-repro\policy
Set-Content bx-policy-repro\Dockerfile "FROM scratch"
Set-Content bx-policy-repro\policy\allow.rego @'
package docker

default allow := true

decision := {"allow": allow}
'@

cd bx-policy-repro
docker buildx build --check --policy "filename=cwd://policy/allow.rego" .

failed to stat policy file cwd://policy/allow.rego: stat policy\allow.rego: invalid argument

The policy loader now converts local and cwd:// policy filenames with filepath.ToSlash and path.Clean before resolving them through the policy filesystem.