perf(cli): Improve concurrency when setting up node_modules and loading cached npm package info

[nathanwhit]

The same issue in two different places - doing blocking FS work in an async task, limiting the amount of work that happens concurrently.

• When setting up node_modules, where we try to set up entries concurrently but were blocking other tasks from actually running.
• When loading package info from the npm registry file cache, loading and deserializing is expensive and prevents concurrency. This was especially noticeable when loading an npm resolution snapshot from a lockfile (snapshot_from_lockfile in deno_npm).

Installing deps in deno-docs:

❯ hyperfine -i -p 'rm -rf node_modules/' '../d7/deno-main i' '../d7/target/release/deno i'
Benchmark 1: ../d7/deno-main i
  Time (mean ± σ):      2.193 s ±  0.027 s    [User: 0.589 s, System: 1.033 s]
  Range (min … max):    2.151 s …  2.242 s    10 runs

Benchmark 2: ../d7/target/release/deno i
  Time (mean ± σ):      1.597 s ±  0.021 s    [User: 0.977 s, System: 1.337 s]
  Range (min … max):    1.550 s …  1.627 s    10 runs

Summary
  ../d7/target/release/deno i ran
    1.37 ± 0.02 times faster than ../d7/deno-main i

Caching npm:@11ty/eleventy:

❯ hyperfine -i -p 'rm -rf node_modules/' --warmup 5 '../../d7/deno-main cache npm:@11ty/eleventy' '../../d7/target/release/deno cache npm:@11ty/eleventy'
Benchmark 1: ../../d7/deno-main cache npm:@11ty/eleventy
  Time (mean ± σ):     129.9 ms ±   2.2 ms    [User: 27.5 ms, System: 101.3 ms]
  Range (min … max):   127.5 ms … 135.8 ms    10 runs

Benchmark 2: ../../d7/target/release/deno cache npm:@11ty/eleventy
  Time (mean ± σ):     100.6 ms ±   1.3 ms    [User: 38.8 ms, System: 233.8 ms]
  Range (min … max):    99.3 ms … 103.2 ms    10 runs

Summary
  ../../d7/target/release/deno cache npm:@11ty/eleventy ran
    1.29 ± 0.03 times faster than ../../d7/deno-main cache npm:@11ty/eleventy

[dsherret]

LGTM!