reconsider common.py

[marc-portier]

there seems to be nothing common about it.

• the insert_** only exists by virtue of the feature in main to preload the store

  • [x] it could move over there (and its tests to test_main)

• the mapping functions between namedgraph uri and travharv tasksnames are apperently at multiple places, but that feels like it should not be the case

  • [ ] those probably should be moved close to the store (refactoring suggested in #20) and
  • [ ] should probably not be used in the context of the config_build (where they do appear?)