fix: correctly load legacy plugins

[cristiand391]

This PR fixes an issue where CLIs that includes @oclif/plugin-legacy and at least one old, invalid plugin would get legacy plugins as the only ones installed.

left: ci-v5 plugin installed, shows as the only plugin (--core should show core plugins too), plugins:inspect webhooks fails even though @heroku-cli/plugin-webhooks is a core plugin.
right: with oclif fix, plugins:inspect is able to find it.

plugin-legacy init hook where invalid, legacy plugins are converted:
https://github.com/oclif/plugin-legacy/blob/main/src/hooks/init.ts

Repro

1. install Heroku CLI: npm i -g heroku
2. check core plugins: heroku plugins --core, this only returns addons-v5 8.1.7 (core) because it's the first invalid plugin found.
3. modify the splice line in the JS code
4. run heroku plugins --core again, should get all core plugins.

[skip-validate-pr]

[cristiand391]

Array.splice mutates the array and returns an array of deleted items in this case, see:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/splice#return_value

so, this.plugins was set to the non-compatible version of the plugin loaded here:

core/src/config/config.ts

Line 193 in a2efc04

await this.loadPluginsAndCommands()

[k80bowman]

Great find, thank you!

[WillieRuemmele]

QA Notes

---

npm i -g heroku

 ➜  heroku plugins --core
addons-v5 8.1.7 (core)

modify /Users/william.ruemmele/.nvm/versions/node/v18.16.0/lib/heroku/node_modules/@oclif/core/config.js L 649 to be

            if (idx !== -1)
                 this.plugins.splice(idx, 1, plugin);
            this.loadCommands(plugin);

instead of

            if (idx !== -1)
                this.plugins = this.plugins.splice(idx, 1, plugin);
            this.loadCommands(plugin);

and then run heroku plugins --core again, and get

 ➜  heroku plugins --core                                                                      
@oclif/plugin-commands 2.2.2 (core)
@oclif/plugin-help 5.2.9 (core)
@oclif/plugin-legacy 1.3.0 (core)
@oclif/plugin-not-found 2.3.16 (core)
@oclif/plugin-plugins 2.4.7 (core)
@oclif/plugin-update 3.1.10 (core)
@oclif/plugin-version 1.3.4 (core)
@oclif/plugin-warn-if-update-available 2.0.29 (core)
@oclif/plugin-which 2.2.8 (core)
addons-v5 8.1.7 (core)
apps 8.1.8 (core)
apps-v5 8.1.8 (core)
auth 8.1.7 (core)
autocomplete 8.1.7 (core)
buildpacks 8.1.7 (core)
certs 8.1.7 (core)
certs-v5 8.1.7 (core)
ci 8.1.7 (core)
ci-v5 8.1.8 (core)
config 8.1.7 (core)
container-registry-v5 8.1.7 (core)
git 8.1.8 (core)
heroku 8.1.8 (core)
local 8.1.8 (core)
oauth-v5 8.1.8 (core)
orgs-v5 8.1.4 (core)
pg-v5 8.1.7 (core)
pipelines 8.1.7 (core)
ps 8.1.7 (core)
ps-exec 2.4.0 (core)
redis-v5 8.1.7 (core)
run 8.1.4 (core)
spaces 8.1.7 (core)
status 8.1.7 (core)
webhooks 8.1.7 (core)

verified some other plugin commands

 ➜  heroku plugins:inspect run
failed
    Error: @heroku-cli/plugin-run not installed

vs

 ➜  heroku plugins:inspect run
└─ @heroku-cli/plugin-run
   ├─ version 8.1.4
   ├─ homepage https://github.com/heroku/cli
   ├─ location /Users/william.ruemmele/.nvm/versions/node/v18.16.0/lib/node_modules/heroku/node_modules/@heroku-cli/plugin-run
   ├─ commands

couldn't find any other plugin:* commands that were broken