piploc install --upgrade (probably) won't work

Just as a point of information, the `--target` option to pip doesn't work with `--upgrade`, so `piploc -U` won't work as expected.

Demonstration:

```
>py -m pip install -t xxx wheel==0.30.0
Collecting wheel==0.30.0
Downloading https://files.pythonhosted.org/packages/0c/80/16a85b47702a1f47a63c104c91abdd0a6704ee8ae3b4ce4afc49bc39f9d9/wheel-0.30.0-py2.py3-none-any.whl (49kB)
100% |████████████████████████████████| 51kB 731kB/s
Installing collected packages: wheel
Successfully installed wheel-0.30.0
>py -m pip install -t xxx -U wheel
Collecting wheel
Using cached https://files.pythonhosted.org/packages/96/ba/a4702cbb6a3a485239fbe9525443446203f00771af9ac000fa3ef2788201/wheel-0.33.1-py2.py3-none-any.whl
Installing collected packages: wheel
Successfully installed wheel-0.33.1
>dir .\xxx

Directory: C:\Work\Scratch\xxx

Mode LastWriteTime Length Name
---- ------------- ------ ----
d----- 01/03/2019 15:49 bin
d----- 01/03/2019 15:49 wheel
d----- 01/03/2019 15:49 wheel-0.30.0.dist-info
d----- 01/03/2019 15:49 wheel-0.33.1.dist-info
```

Comece reproduzindo os dois comandos pip da issue e compare-os com o comportamento de piploc -U. Rastreie como piploc lida com a opção de atualização e o diretório de destino; considera-se concluído quando a atualização remove ou substitui os metadados antigos do pacote, em vez de deixar wheel-0.30.0 e wheel-0.33.1 instalados ao mesmo tempo.

Escrita pelo modelo de indexação a partir do texto da issue.