Benchmark info is broken / missing / misleading

[itingliu]

The info about building and running Benchmarks is broken / missing / misleading.

SWIFTCI_USE_LOCAL_DEPS=.. swift package benchmark

doesn’t work, it needs to be ../.. — and then (because the top-level Package.swift also uses SWIFTCI_USE_LOCAL_DEPS, the top-level project also wants to use local dependencies.

I worked around it with local modificatoins to the Package.swift, but this could use some TLC.

Originally posted by @danieleggert in #1198 (comment)