Don't warn about /simple/ fallback for PyPI

and display a little more info about the index used.

##### Before

```
$ pip install thispackagedoesntexist
Collecting thispackagedoesntexist
  DEPRECATION: One or more of your dependencies required using a deprecated fallback to looking at /simple/ to discover it's real name. It is suggested to upgrade your index to  support normalized names as the name in /simple/{name}.
  Could not find any downloads that satisfy the requirement thispackagedoesntexist
  No distributions at all found for thispackagedoesntexist
```

##### After

```
$ pip install thispackagedoesntexist
Collecting thispackagedoesntexist
  Could not find any downloads that satisfy the requirement thispackagedoesntexist
  No distributions at all found for thispackagedoesntexist
```

With a custom index:

```
$ pip install -i https://devpi.mycompany.com/simple thispackagedoesntexist
Downloading/unpacking thispackagedoesntexist
  DEPRECATION: Failed to find 'thispackagedoesntexist' at https://devpi.mycompany.com/simple/thispackagedoesntexist/. It is suggested to upgrade your index to support normalized names as the name in /simple/{name}.
  Could not find any downloads that satisfy the requirement thispackagedoesntexist
  No distributions at all found for thispackagedoesntexist
```

Arguably, the last case is still not ideal, as it didn't find the package even after looking at `/simple/` so the deprecation message is kind of useless and distracting. Maybe that can be solved later.