"dbt --version" should also emit plugin versions

[beckjake]

Describe the feature

dbt --version currently outputs the currently installed dbt version (and the most recently released one). It should also output the currently installed plugin versions.

Currently:

installed version: 0.16.0
   latest version: 0.16.0

Up to date!

I want:

installed dbt version: 0.16.0
   latest dbt version: 0.16.0

Plugins:
  - dbt-postgres: 0.16.0
  - dbt-redshift: 0.16.0
  - dbt-bigquery: 0.16.0
  - dbt-snowflake: 0.16.0

Up to date!

This feels kind of silly for the built-in plugins, but:

• someday we're going to split the plugins that currently live in "core" out
• this will make debugging third-party/external plugins much easier. I don't want to ask people to run pip freeze | grep dbt-spark to find out what version of the spark plugin they're on.

I think Up to date! will probably only refer to dbt-core, but I could be convinced otherwise. Extending it beyond core is probably more work, if we're worried about ambiguity.

the dbt-spark/dbt-presto adapters will have a dbt/adapters/{pluginname}/__version__.py - we should extend that pattern to the plugins that ship with core and use it.

Describe alternatives you've considered

Leave it as-is?

Who will this benefit?

What kind of use case will this feature be useful for? Please be specific and provide examples, this will help us prioritize properly.

• developers/maintainers/people supporting external plugins
• users who want to know what version of a plugin they're running

[sumanau7]

@beckjake Let me know what you think ?
As a side note, when doing local development which branch should we take a checkout from ?