dbt --version reports no plugins

[beckjake]

Describe the bug

When dbt --version is run from a package installed via pip, no plugins are reported.

Steps To Reproduce

pip install dbt-core==0.17.0b1 dbt-snowflake==0.17.0b1
dbt --version

Expected behavior

I expect to see plugin versions!

System information

Which database are you using dbt with?

• postgres
• redshift
• bigquery
• snowflake
• other (specify: ____________)

The output of dbt --version:

$ dbt --version
installed version: 0.17.0-b1
   latest version: 0.16.1

Your version of dbt is ahead of the latest release!

Plugins:

The operating system you're using:
macos

The output of python --version:
Python 3.8.2

Additional context

I think the issue here is that when pip installs multiple PEP420 packages they all end up in a single folder, unlike when you install locally in editable mode. So we need to teach --version to look at all the plugins in all the folders.