`wally install` is destructive

[grilme99]

I'm working on a package that is structured like a monorepo, where all source files are stored in a packages/ directory (lowercase!). However, when I run wally install, it nukes the entire packages folder instead of warning me (or doing nothing, because Wally writes to uppercase Packages).

Wally should detect if packages contains anything but dependencies (maybe check if it has an _Index folder?) and warn me before deleting the folder. This could easily cause people to lose entire projects.

I am on macOS.