[NFC] Remove `Project.inPlace` to simplify package init logic

[WowbaggersLiquidLunch]

Project.inPlace is used only once, when initialising a new package through SwiftPM. And it's somewhat redundant along with .name when .path is really the only thing needed.

The logic can be simplified even further if/when we use libSwiftPM directly.

This PR contains no functional change, but is API-breaking for any other project that depends on CartonKit or SwiftToolchain.

[WowbaggersLiquidLunch]

@carson-katri I think you are the previous author of most of the things changed in this PR. Could you take a look at them and see if everything is done properly? Thanks!

[WowbaggersLiquidLunch]

Althouth the PR passed all CI tests, somehow it didn't pass some tests locally:

However, the test failures only appear in Xcode's report navigator, but not issue navigator. I'm not sure why this is happening.

[MaxDesiatov]

Hi @WowbaggersLiquidLunch, would you be interested in picking this up again in the near future?

[WowbaggersLiquidLunch]

Hi @MaxDesiatov Sorry for the delay in my reply and the long time since I last worked on this.

I plan on revisiting this PR. I'm not able to do it right now, but I think I'll do it within about a month. It's one of my backlogged things that I want to pick back up over summer.

If you prefer, I can close this PR for now, and reopen it later when I start working on it again.

[kateinoigakukun]

We no longer maintain init subcommand because SwiftPM's init subcommand is already WebAssembly compatible. Anyway, thank you for your contribution!