Allow renaming the autogenerated Kotlin package name

[dfabulich]

The app module package is autogenerated and unconfigurable.

What is this autogenerated based on?

The lower-cased and de-camel-cased module name. E.g., FooBar becomes foo.bar, and SomeBIGThing becomes some.big.thing.

(Why isn't it changeable? How is it even possible to have an unchangeable setting??)

We've always thought that derived package name customization would be a useful feature, but (to my surprise) no-one has ever requested it, and so we never bothered implementing it.

Originally posted by @marcprux in #323

[dfabulich]

In Slack, Marc suggested that a PR might start with:

https://github.com/skiptools/skipstone/blob/3b07536d9acb16ca9f1db895f0728883f93af5e8/Sources/SkipSyntax/Kotlin/KotlinTranslator.swift#L21

There's also a package: option for skip.yaml that I think isn't hooked up to anything.

https://github.com/skiptools/skipstone/blob/3b07536d9acb16ca9f1db895f0728883f93af5e8/Sources/SkipBuild/SkipConfig.swift#L27-L28

And any customization to the package name would probably need to be stored in the serialized KotlinCodebaseInfo.swift so that dependent modules would be able to use the correct name for the module.

There may be a lot of corner cases that need to be handled, but fortunately SkipSyntax has a lot of test cases that would help iterate on things quickly (or at least quicker than testing it by running against real modules and then debugging places where the customized package name is not correct).