UPDATE: Improve build system

[T-K-233]

This PR changes the build system to support the design idea of "A Balance between Flat Instantiation and Dependency Injection".

The goal is to shape the system to use Verilog-like flat instantiation and connection description of modules, but at the same time supports Chipyard-like configuration class-based parameter descriptions.

The main changes are:

• separate modules into different Scala packages at the top level (ideally this should be in a folder like generators/, but I could not configure mill to find package contents in a subfolder).
• add config-based parameter configuration for modules.
• isolate Chisel builder as a separate package.
• improve tcl script generation logic.
• use "designs" as the buildable unit.
• add CI runner on mill to check Vivado build flow.