We already implemented in essence the rule suggested in lampepfl/dotty-feature-requests#14:

```
Γ ⊨ p : T
------------------------ (Sel-<:-Proj)
Γ ⊨ p.A <: T#A
```

This rule is implemented in `isSubPrefix`. But we did not get to check that since we concluded prematurely that
an alias type would never match. The alias type in question in i17064.scala was

```scala
Outer#Inner
```
Since `Outer` is not a path, the asSeenFrom to recover the info of `Outer#Inner this got approximated with
a `Nothing` argument and therefore the alias failed. It is important in this case that we could still succeed
with a `isSubPrefix` test, which comes later. So we should not return `false` when the prefix is not a singleton.

Fixes scala#17064

`NoPrefix.isStable` already returns true.