Exclude pip modules from reshim?

Hi there, I was wondering if it's possible to prevent some installed pip modules from being linked during a `asdf reshim python`.

For example, I have the `python-apt` module installed because I write python scripts to interact with apt, however this now overrides my base OS version's `apt` binary

I don't want to totally prevent python modules from being installed in my path because of things like `ansible`

```
~
$ apt --version
1.18

~
$ apt install apt
* Running: sudo apt-get install apt
Reading package lists... Done
Building dependency tree
Reading state information... Done
apt is already the newest version (2.0.9).
The following packages were automatically installed and are no longer required:
libfwupdplugin1 libxmlb1
Use 'sudo apt autoremove' to remove them.
0 upgraded, 0 newly installed, 0 to remove and 20 not upgraded.

~
$ which apt
/home/ddastoor/.asdf/shims/apt

~
$ pip3 list | grep apt
apt-wrapper 1.18
pyapt 0.0.6
wrapt 1.15.0
```

まず、`python-apt` をインストールした状態で `asdf reshim python` を再現し、`apt` shim がどのように生成されるかを調べます。この変更では、`ansible` のような有用なコマンドのリンクを維持しながら、pip が提供する特定のコマンドを除外できる方法を用意し、システムの `apt` は引き続き利用できるようにする必要があります。

索引モデルが issue の本文から書いたものです。