track local changes with overlayfs

[milahu]

feature request:
allow to change/edit/patch local package installs
by auto-mounting the original package with overlayfs

this should happen 'behind the scenes'
so when i start editing files, the original file is preserved somewhere

todo:
benchmark performance of overlayfs
when thousands of directories are mounted
(probably not a good idea)

expensive alternative:
write a custom filesystem driver (pnpmfs)
that listens to read/write requests
and only on a write request,
the overlayfs is mounted

cheap alternative:
mount the pnpm global store with overlayfs
so all user edits go to one overlay
= allow global edits while tracking changes
pnpm installs go to the lower layer of the overlay

background:
i want to patch installed packages
and keep track of changes
so i can generate a distributable patch file

https://github.com/ds300/patch-package
does this by downloading the original package from npm servers
and comparing original vs patched version

this re-download should be skippable
as long as it can be done in a performant way

also probably a challenge to do that on windows ....

slightly related, cos docker is using overlayfs
issue #1637 pnpm monorepo docker support