change source package path from io.kixi.* to io.kixi.core.*

[SingingBush]

I intend to setup JPMS for both Ki Core and Ki KD but as a precursor it would be wise to make sure that the package name not be to broad. The reason being that split packages are not allowed with the Java module system. Therefore this PR is to move all code in this project from io.kixi.* to io.kixi.core.*.

As this is a breaking change, I'll wait for you to confirm that you're happy to proceed before merging. Once done then the module-info should be something like:

module kixi.ki.core {
    requires kotlin.stdlib;
    requires kotlin.stdlib.jdk8;

    exports io.kixi.core;
    exports io.kixi.core.text;
    exports io.kixi.core.uom;
}