fix(websockets): require @nestjs/platform-socket.io correctly

[merceyz]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

The undeclared peer dependency @nestjs/platform-socket.io is required from @nestjs/core instead of @nestjs/websockets

Issue Number: N/A

What is the new behavior?

The peer dependency is declared and required from @nestjs/websockets

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

See https://yarnpkg.com/advanced/rulebook#packages-should-only-ever-require-what-they-formally-list-in-their-dependencies
Similar to #5477

[coveralls]

Pull Request Test Coverage Report for Build 6f9f87d2-06ea-4c9a-976f-de9b11c5bb49

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.741%

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Totals
Change from base Build fdd57427-3ec5-473b-b2a7-c14bbe01124b:	0.0%
Covered Lines:	5080
Relevant Lines:	5362

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💛 - Coveralls

[kamilmysliwiec]

Thanks for your contribution! We should add the peerDependenciesMeta and mark this dependency as optional (+ we should add @nestjs/platform-ws too)

[merceyz]

I didn't mark it as optional because it didn't look like it was.

(+ we should add @nestjs/platform-ws too)

Could you give me a pointer to where it's used / be more specific?