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
```

先重現 issue 中的兩個 pip 指令，並將它們與 piploc -U 的行為進行比較。追蹤 piploc 如何處理升級選項和目標目錄；完成的標準是升級會移除或取代舊的套件中繼資料，而不是讓 wheel-0.30.0 和 wheel-0.33.1 同時保持已安裝狀態。

由索引模型根據 Issue 內容生成。