Tidy up init command impl, make it unpack into current folder

[lihaoyi]

Unpacking into the current folder is much more useful due to the fact that we recommend people use a bootstrap script.

• With the previous approach, you end up with a bootstrap script in the parent dir and a subfolder with another bootstrap script with the real build inside.
• With the new approach, you download a bootstrap script, run init, and it fleshes out the example in place

Since we now unpack into an existing folder that may or may not have things already in it, we add a new heuristic to fail early if the unpacking would over-write any existing files. This is a compromise, since we cannot easily assume the folder is 100% empty (what if there's a .DS_STORE in it???), nor can we remove any previous files we do not understand (what if there's already a .gitignore or .git?), and we also do not want to stomp over any existing files (what if there's already a build.mill and someone accidentally calls init?).

Tweak the implementation to avoid using so many nested Trys, normal exception handling is enough

Tested manually via ./mill installLocalCache

[lihaoyi]

CC @pawelsadlo

[lefou]

There should be at least a message/warning for every file we delete or overwrite.

[lihaoyi]

@lefou with the current PR impl, we do not delete/overwrite any file, and fail early if there's any conflict between the zip example and the files already on disk

[lihaoyi]

There's something funky with ./mill -iw main.init.test looping indefinitely, but that's already present in master and not unique to this PR. Will look into it

[lefou]

I was referring to the deletion of a pre-existing mill script.

mill/main/init/src/mill/init/InitModule.scala

Line 77 in 49a2811

os.remove(T.workspace / "mill")