backstage-cli: lint command should lint a package/plugin's dependencies with custom rules

Feature Suggestion

When backstage-cli lint is run in a package or plugin, it should take a look at the dependencies and throw warnings/errors based on some rules.

1. If you are importing packages which should not be imported, the backstage instance should log a warning. (e.g. @backstage/core should also not be imported docs).
2. Frontend packages should not be imported in backend packages/plugins and vice versa. (e.g. there are two separate Discovery APIs in separate packages for frontend/backend)

(Future scope) 3. (?) Suggestions on some packages and their dependencies based on ADRs. For example, using luxon over any other date time library. https://github.com/backstage/backstage/issues/3401

Possible Implementation

Challenging but depends on each rule. Having a “Disallowed list” of dependencies is one way to do (1).

Context

Discussion https://github.com/backstage/backstage/pull/4069#issuecomment-760038965

[Rugvip]

@TejasQ There’s nothing that definitely tells you if it’s a frontend or backend package. As mentioned on slack it’s possible to check the extension of the publishConfig.main and publishConfig.module field in package.json, but that’s not a completely reliable check either as it may change with custom build setups etc. It’s probably best check we have for now though, and since the lint rule is easy to disable for cases where it doesn’t fit I’m thinking it would be a good addition.

[TejasQ]

I shall create an ESLint plugin for backstage since we’ll have more needs arise eventually. Already started. Here’s what I have so far: