Only track client entry modules in the client reference manifest (#48814)
This is currently an overhead, that we check a module's layer (`mod.buildInfo.rsc?.type === RSC_MODULE_TYPES.client`) and put all client modules in the client reference manifest, but the manifest is only used for accessing these entry modules. So here we change the util to check if it's an client entry instead.
With this change the client manifest of a test app decreased from 177 KB to 70 KB.
Ref: https://github.com/vercel/next.js/blob/5b609e264f56e6e7c6e230d88ad444650b5b6ba9/packages/next/src/build/analysis/get-page-static-info.ts#L50-L64