perf(turbopack): Merge nodes with same starting point (#76938)
### What?
Implement one more trivial graph optimization
### Why?
It reduces the number of modules marginally in real world apps.
I posted the result at https://vercel.slack.com/archives/C04RPSD7EPM/p1732095860455549?thread_ts=1732091170.097799&cid=C04RPSD7EPM
### How?
Closes PACK-3442
