fix: refactor index.ts to separate files for files & integrity operations

[mmaietta]

Notes:

• Refactor functions out of index.ts into dedicated files (file utils & integrity generator)
• Code change: asar.listPackage was missing a 2nd argument. I checked with electron/asar and its default behavior is isPack: false (https://github.com/electron/asar/blob/c2a5f3a59ba8e72365e9dc9a3cf3ad606f21734b/src/filesystem.ts#L188)
• Extracted functions:
  • copyAndShimAsarIfNeeded
  • copySnapshots
  • lipoMachOFiles
  • mergeAppFiles
  • injectAsarIntegrity

This is to streamline fixing some bugs and make the PRs easier to read and understand: #116 #117
Plus, I'd like to contribute to the test suite, which will be easier to do with this refactor.

[mmaietta]

FWIW, this is decoupled from #122, but since that PR is completely a test suite upgrade, it may be best to do #122 first, then I rebase this PR on to the upgraded test infrastructure