[tools,topgen] Split topgen into creating complete topcfg and the rest

Description

Topgen is a monolithic tool and is called multiple times to generate the full set of artifacts. Each time it generated the complete top configuration, and then performs a some subset of all the actions it is able to do. This is not efficient and is hard to maintain.

This calls for a change in the topgen flow so the complete top configuration is generated and written to disk by one tool. The actions that depend on the complete config can then be performed by smaller separate tools, and most importantly, they can be invoked only if the complete top configuration file has changed. This will allow a more incremental flow, and since most of the separate tasks depending on the complete config are independent, they can be done in parallel.

This is not only more efficient and simpler, but it simplifies the development of different tops, so can be very helpful for multi-top.