pdm plugin list shows duplicate results if pdm is installed through pipx

[xulongwu4]

• I have searched the issue tracker and believe that this is not a duplicate.

Make sure you run commands with -v flag before pasting the output.

Steps to reproduce

1. Install pdm using pipx: pipx install pdm
2. Install pdm-venv: pipx inject pdm pdm-venv
3. Check the list of installed plugins: pdm plugin list

Actual behavior

Installed plugins:
pdm-venv 0.2.0
    A plugin for pdm that enables virtualenv management
pdm-venv 0.2.0
    A plugin for pdm that enables virtualenv management

The plugin shows up twice in the list.

Expected behavior

Installed plugins:
pdm-venv 0.2.0
A plugin for pdm that enables virtualenv management

## Environment Information

```bash
# Paste the output of `pdm info && pdm info --env` below:

This was run from outside a project so pdm info is not applicable.

$ pdm --version
pdm, version 1.7.1