perf: load binaries in the same process

[paul-soporan]

What's the problem this PR addresses?

While looking into improving Yarn's startup time, we've observed the fact that corepack adds extra overhead by spawning a new process to run the invoked binary.

How did you fix it?

This PR makes corepack load binaries in the same process, removing the spawn indirection and considerably improving the startup time:

package-manager --version	Before	After
yarn@3.2.0	183.1 ms	159.7 ms
pnpm@6.29.1	92.0 ms	69.8 ms
npm@8.1.0	160.8 ms	131.6 ms

The way it achieves this is by modifying process.{argv,execArgv} and process.mainModule (and the module cache to make sure that require.main === module keeps working) to trick the binary into thinking that it was spawned directly instead of having been loaded into the current process.

It might feel a bit magical at first, but we feel it's worth it.