implement "plugin" package type (#149370)

This PR updates the core discovery logic to support loading plugins from
packages. This logic is additive, so that the existing plugins in the
repo and third-party plugins can continue to be loaded via the existing
mechanism, but with https://github.com/elastic/kibana/pull/148130 we
will be automatically migrating all plugins in the repo to packages,
which will use this logic.

The logic is already in-use in that PR, and was developed there, but
extracted here for easier review.

The logic is relatively simple, where a list of packages in the repo are
attached to the core `Env` and then filtered by core before converting
all plugin packages to `PluginWrapper`. The `PluginWrapper` still
exposes the plugin manifest to the rest of the code, and it is used in
many places, so rather than making changes to the `PluginWrapper` I'm
faking a legacy plugin manifest with the plugin package manifest.

@elastic/kibana-core: I'm going to need some help identifying what we
need to get test coverage for. This is a pretty simple addition to the
core IMO, and if it didn't work then nothing would work, so I'm pretty
confident in it, but would still appreciate your feedback.

packages/kbn-cli-dev-mode/src/bootstrap.ts

@@ -7,6 +7,7 @@
  */
 
 import { REPO_ROOT } from '@kbn/repo-info';
+import { getPackages } from '@kbn/repo-packages';
 import { CliArgs, Env, RawConfigAdapter } from '@kbn/config';
 import { CliDevMode } from './cli_dev_mode';
 import { CliLog } from './log';
@@ -25,6 +26,7 @@ export async function bootstrapDevMode({ configs, cliArgs, applyConfigOverrides
   const env = Env.createDefault(REPO_ROOT, {
     configs,
     cliArgs,
+    repoPackages: getPackages(REPO_ROOT),
   });
 
   const config = await loadConfig({

packages/kbn-cli-dev-mode/tsconfig.json

@@ -24,6 +24,7 @@
     "@kbn/repo-source-classifier",
     "@kbn/import-resolver",
     "@kbn/picomatcher",
+    "@kbn/repo-packages",
   ],
   "exclude": [
     "target/**/*",