Refactor attrs support into reusable code

[tristanlatr]

This will help build code to support dataclasss and namedtuple without too much preliminary refactors.

[codecov[bot]]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (f03223c) 92.68% compared to head (b90beba) 92.66%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #719 +/- ## ========================================== - Coverage 92.68% 92.66% -0.03% ========================================== Files 47 48 +1 Lines 8180 8231 +51 Branches 1959 1970 +11 ========================================== + Hits 7582 7627 +45 - Misses 343 347 +4 - Partials 255 257 +2 ``` | [Files](https://app.codecov.io/gh/twisted/pydoctor/pull/719?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=twisted) | Coverage Δ | | |---|---|---| | [pydoctor/astutils.py](https://app.codecov.io/gh/twisted/pydoctor/pull/719?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=twisted#diff-cHlkb2N0b3IvYXN0dXRpbHMucHk=) | `94.67% <100.00%> (+0.98%)` | :arrow_up: | | [pydoctor/extensions/\_dataclass\_like.py](https://app.codecov.io/gh/twisted/pydoctor/pull/719?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=twisted#diff-cHlkb2N0b3IvZXh0ZW5zaW9ucy9fZGF0YWNsYXNzX2xpa2UucHk=) | `95.34% <95.34%> (ø)` | | | [pydoctor/extensions/attrs.py](https://app.codecov.io/gh/twisted/pydoctor/pull/719?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=twisted#diff-cHlkb2N0b3IvZXh0ZW5zaW9ucy9hdHRycy5weQ==) | `91.66% <87.87%> (-7.23%)` | :arrow_down: |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.