fix: support the ng-package.json in secondary entry points

[klemenoslaj]

Currently secondary entry points are breaking no-implicit-dependencies TSLint rule, because the
rule is checking for package.json files, which in secondary entry points are anyway just containing
ngPackage configuration. Adding the support for ng-package.json removes the need for package.json,
thus TSLint will look correctly in parent package.json file and act accordingly.

fix 1391

I'm submitting a...

[x] Bug Fix
[ ] Feature
[ ] Other (Refactoring, Added tests, Documentation, ...)

Checklist

• Commit Messages follow the Conventional Commits pattern
  • A feature commit message is prefixed "feat:"
  • A bugfix commit message is prefixed "fix:"
• Tests for the changes have been added

Description

Fixes: 1391

Does this PR introduce a breaking change?

[ ] Yes
[x] No

[klemenoslaj]

I as well wanted to add an addition to the documentation. Would it be fine like the example bellow?

docs/secondary-entrypoints.md

### Alternative to `package.json`

Alternatively, you could create `ng-package.json` instead of `package.json`.
This is particularly useful in conjunction with `no-implicit-dependencies` TSLint rule, which will complain if `package.json` does not contain the dependencies used in the secondary entry point, which is misleading since all the dependencies should be mentioned in the primary `package.json`.

[klemenoslaj]

@alan-agius4, how do you prefer in this projects for review adjustments to be applied? With additional commit or fixing the current commit?

If we're squashing it anyway makes no difference, just wanna be sure.

[alan-agius4]

@klemenoslaj, doesn't make a difference :)

[alan-agius4]

Thanks LGTM

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