`ufs.copyFileSync` fails when src and destination folders exist in separate volumes

[iamogbz]

Description

Copying a file from an existing location in one fs module into an existing location in another fs module fails

Steps to reproduce

With file

const fs = require('fs')
const path = require('path')

const { vol } = require('memfs')
const { ufs } = require('unionfs')

ufs.use(vol).use(fs)

const file1 = 'testfile.txt'
const file2 = 'somefolder/testfile.txt'

try {
    vol.mkdirSync(path.dirname(file2), { recursive: true })
    fs.writeFileSync(file1, '')
    ufs.copyFileSync(file1, file2)
} finally {
    if (fs.existsSync(file1)) fs.unlinkSync(file1)
}

Results

./node_modules/unionfs/lib/union.js:636
                    throw err;
                    ^

Error: ENOENT: no such file or directory, open 'testfile.txt'
    at createError (./node_modules/memfs/lib/volume.js:130:17)
    at Volume.openFile (./node_modules/memfs/lib/volume.js:818:15)
    at Volume.openBase (./node_modules/memfs/lib/volume.js:822:25)
    at Volume.openSync (./node_modules/memfs/lib/volume.js:833:21)
    at Volume.readFileBase (./node_modules/memfs/lib/volume.js:919:23)
    at Volume.readFileSync (./node_modules/memfs/lib/volume.js:934:21)
    at Volume.copyFileBase (./node_modules/memfs/lib/volume.js:1113:24)
    at Volume.copyFileSync (./node_modules/memfs/lib/volume.js:1127:21)
    at Union.syncMethod (./node_modules/unionfs/lib/union.js:630:35)
    at Union.this_1.<computed> [as copyFileSync] (./node_modules/unionfs/lib/union.js:525:34) {
  code: 'ENOENT',
  prev: Error: ENOENT: no such file or directory, copyfile 'testfile.txt' -> 'somefolder/testfile.txt'
      at Object.copyFileSync (fs.js:1801:3)
      at Union.syncMethod (./node_modules/unionfs/lib/union.js:630:35)
      at Union.this_1.<computed> [as copyFileSync] (./node_modules/unionfs/lib/union.js:525:34)
      at Object.<anonymous> (./example.js:16:9)
      at Module._compile (internal/modules/cjs/loader.js:936:30)
      at Object.Module._extensions..js (internal/modules/cjs/loader.js:947:10)
      at Module.load (internal/modules/cjs/loader.js:790:32)
      at Function.Module._load (internal/modules/cjs/loader.js:703:12)
      at Function.Module.runMain (internal/modules/cjs/loader.js:999:10)
      at internal/main/run_main_module.js:17:11 {
    errno: -2,
    syscall: 'copyfile',
    code: 'ENOENT',
    path: 'testfile.txt',
    dest: 'somefolder/testfile.txt',
    prev: null
  }
}

[alexvgjm]

#747 Just made a pull request