fix: remove broken cdk alias entrypoint metadata

[Pugsin]

Summary

• set the cdk alias package entrypoint to an empty string in projen config
• remove generated main and types fields from packages/cdk/package.json
• keep the existing CLI shim bin/cdk and aws-cdk runtime dependency

Why

The published cdk@2.1128.0 package is a CLI alias package. Its tarball includes bin/cdk, but does not include lib/index.js or lib/index.d.ts, while package metadata advertises those files through main and types.

Validation

• Checked npm pack cdk@2.1128.0: tarball includes bin/cdk, package metadata points to missing lib/index.js / lib/index.d.ts
• Checked projen NodePackageOptions: entrypoint: '' omits main from package metadata
• Verified PR diff only removes the broken generated metadata and adds the projen source setting

[mrgrain]

@Pugsin looks good but you'll need to include the contribution statement. Best to always read the pull request template of a repo you are contributing to.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.73%. Comparing base (7daa104) to head (2a2a906).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1643   +/-   ##
=======================================
  Coverage   88.73%   88.73%           
=======================================
  Files          77       77           
  Lines       11365    11365           
  Branches     1588     1588           
=======================================
  Hits        10085    10085           
  Misses       1250     1250           
  Partials       30       30           

Flag	Coverage Δ
suite.unit	88.73% <ø> (ø)

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