Tab complete on a command with dash suggests a function that has a dash in the name

# Steps to reproduce

```powershell
function a-b {}
foo -<tab>
```

# Expected behavior

```none
Nothing since `foo` doesn't exist
```

# Actual behavior

```none
foo a-b
```

Regression from WinPS5.1.  Repros in 6.2 and 7 Preview4.