vm: perform lazy-loading at top level

[avivkeller]

As it currently stands, the lazy loading for lib/internal/vm/module.js's require of internal/util/inspect is performed in a function. This means that a re-call of the function will be a re-call of the require of the module.

This PR moves the lazy-loading to only occur once.

[atlowChemi]

@RedYetiDev How big a difference is it considering the require cache?

[avivkeller]

top_level.js

const vm = require('vm');

for (let i = 0; i < 1_000_000; i++) {
    vm;
    1+1;
}

vs

in_func.js

for (let i = 0; i < 1_000_000; i++) {
    const vm = require('vm');
    1+1;
}

I got:

Benchmark 1: node in_func.js
  Time (mean ± σ):     218.0 ms ±  10.2 ms    [User: 219.2 ms, System: 12.4 ms]
  Range (min … max):   201.5 ms … 228.3 ms    14 runs
 
Benchmark 2: node top_level.js
  Time (mean ± σ):      17.9 ms ±   0.5 ms    [User: 12.1 ms, System: 6.9 ms]
  Range (min … max):    17.0 ms …  21.1 ms    162 runs
 
Summary
  node top_level.js ran
   12.20 ± 0.66 times faster than node in_func.js

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Note this benchmark is in no way accurate, it's just a basic demo, and it was really just put together in 10 seconds.