Project-relative path support

[danpalmer]

Currently the plugin requires a full path to an alternative SwiftFormat binary. This means that the Nova project configuration will include things like the user's home directory in the path, making this unsuitable for sharing across a team.

If project-relative paths were supported, then a binary in the project could be used instead. This also allows for the binary to be a dependency of a Swift project, e.g. .build/release/swiftformat.