debug: debug incorrectly imports plugin code

[vince-fugnitto]

Bug Description:

The pull-request #10163 incorrectly leaks plugin code to @theia/debug.

theia/packages/debug/src/node/inline-debug-adapter.ts

Line 19 in 2faa18b

import * as theia from '@theia/plugin';

No internal extension should explicitly depend on the plugin- system, we should refactor to implement the feature properly.

@tsmaeder was there a specific reason to directly depend on the plugin types from debug?

[tsmaeder]

I can't think of a reason right off the bat. Can we just move the class to the theia plugin package? It just seems to be a wrapper around the Theia inline debug adapter.