Swiftly use should be able to detect that my folder has a Package.swift

[matthewbastien]

Description:
Trying to run swiftly use <version> from within a newly created Swift package will not create a .swift-version file. I have to either create the file myself or initialize a git repository before invoking Swiftly.

Steps to reproduce:

1. Create a folder for a new Swift package.
2. Initialize the package with swift package init.
3. Run swiftly use 6.2.0 from within the newly created and initialized folder.

Swiftly will then update the global default toolchain to 6.2.0 without creating a .swift-version file.

Expected Behavior:

Swiftly should ask me for permission to create a .swift-version file in the same directory as my Package.swift.