fix: lazy load of which module (#176)

Currently `@npmcli/git` is eagerly getting and caching the `git` path in
a user's system which can have a significant performance cost when
requiring `@npmcli/git`. This PR avoids this cost by lazily getting &
caching `git` path on first use.

lib/spawn.js

@@ -2,10 +2,10 @@ const spawn = require('@npmcli/promise-spawn')
 const promiseRetry = require('promise-retry')
 const log = require('proc-log')
 const makeError = require('./make-error.js')
-const whichGit = require('./which.js')
 const makeOpts = require('./opts.js')
 
 module.exports = (gitArgs, opts = {}) => {
+  const whichGit = require('./which.js')
   const gitPath = whichGit(opts)
 
   if (gitPath instanceof Error) {