Rework reparenting in post processing

[tristanlatr]

This PR probably introduces more changes than necessary... Still WIP

[codecov[bot]]

Codecov Report

Patch coverage: 89.18% and project coverage change: -0.05 :warning:

Comparison is base (965ed95) 92.62% compared to head (df4d2f6) 92.57%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #724 +/- ## ========================================== - Coverage 92.62% 92.57% -0.05% ========================================== Files 47 47 Lines 8132 8185 +53 Branches 1944 1956 +12 ========================================== + Hits 7532 7577 +45 - Misses 343 349 +6 - Partials 257 259 +2 ``` | [Impacted Files](https://app.codecov.io/gh/twisted/pydoctor/pull/724?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=twisted) | Coverage Δ | | |---|---|---| | [pydoctor/model.py](https://app.codecov.io/gh/twisted/pydoctor/pull/724?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=twisted#diff-cHlkb2N0b3IvbW9kZWwucHk=) | `94.40% <83.33%> (-0.24%)` | :arrow_down: | | [pydoctor/astbuilder.py](https://app.codecov.io/gh/twisted/pydoctor/pull/724?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=twisted#diff-cHlkb2N0b3IvYXN0YnVpbGRlci5weQ==) | `95.89% <90.32%> (-0.50%)` | :arrow_down: |

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