[tools,topgen] Simplify topgen

This is a big change in topgen. Some salient changes:
- It always regenerates from scratch the ipgen in-memory configurations
  (as IpBlock objects) so it is truly incremental: the previous flow
  reused the pre-existing generated ipgen hjson if found.
- It delays the actual ipgen file generation until the complete top config
  is generated. This is a step towards breaking topgen into smaller and
  mutually independent steps, thus enabling parallelizing them.
- It converges with a single pass of process_top.
In order to accomplish this some important changes are made:
- Call some of the merge_top functions suitable for each specific ipgen
  prior to generating the in-memory hjson config. This means not all
  IpBlock objects are available, so these merge functions are instrumented
  to skip blocks missing.
- The generation of ipgens are ordered according to dependencies. This
  order created from the expected block functionality, so is not derived
  from a constructed graph, but converging in one pass means it is correct.
- Calling merge functions early means some of the objects in the in-memory
  top config are not plain dictionaries but are classes, so the code needs
  to handle either.

Part of lowRISC#25920

Signed-off-by: Guillermo Maturana <[email protected]>

hw/top_darjeeling/ip_autogen/pwrmgr/data/top_darjeeling_pwrmgr.ipconfig.hjson

@@ -29,6 +29,7 @@
         module: soc_proxy
       }
     ]
+    NumRstReqs: 2
     rst_reqs:
     {
       int:
@@ -68,7 +69,6 @@
         }
       ]
     }
-    NumRstReqs: 2
     wait_for_external_reset: true
     NumRomInputs: 3
     top_pkg_vlnv: lowrisc:constants:top_darjeeling_top_pkg