* Deprecate -[FlutterDartProject initFromDefaultSourceForConfiguration] (#18886)

`-[FlutterDartProject initFromDefaultSourceForConfiguration]` no
longer seems very useful.  It calls `-initWithPrecompiledDartBundle:`
or `-initWithFlutterAssets:dartMain:packages:`, but since it now
passes `nil` for all arguments, both paths end up doing the same
thing.

Additionally, `-initFromDefaultSourceForConfiguration` is awkward to
use in Swift.  The automatically generated Swift interface is:

    public convenience init!(fromDefaultSourceForConfiguration: ())

and it's not obvious how to call that.

Let's deprecate `-initFromDefaultSourceForConfiguration` and instead
expect callers to use the existing `-init` method. (We can make
`-init` do different things for different build configurations later
if necessary.)

Bonus: Rename some parameters to make it more obvious when they may
be `nil`.