perf(@angular/ssr): prevent potential stampede in entry-points cache

[jkrems]

If multiple concurrent requests hit getEntryPointExports, all of
them would previously see the cache miss for entry point. With this
change, only the first request will and the others can leverage the
cache.

This can be important when instances are added to a pool under high
traffic.

PR Checklist

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• The commit message follows our guidelines: https://github.com/angular/angular-cli/blob/main/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
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PR Type

What kind of change does this PR introduce?

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

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

[jkrems]

Feel free to close as "not worth it". I'm just having flashbacks to issues with cache stampedes during bootup, so I'm a bit overly sensitive about it. :)

[alan-agius4]

LGTM, one tiny nit.

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