change cherry picked from python module template
commit 71b117ba0cf9f1251b1cac26d0994df03a4c787d

Related T703

Related T703

This is a work:
- to reduce logic duplication in loader-dir, tar, svn.
- towards the unification between the loader-git's base loader and the
  other loaders: dir, tar and svn.

Related T613

Related T613

Related T613

Example: loader-svn/loader-debian are forced to send data for technical reasons

The load function should be the one be provided by subclass

This is also a step towards merging swh.loader.git.base.BaseLoader.

This is also a step towards merging swh.loader.git.base.BaseLoader.

Currently, it is the task's job to open/close fetch_history.
After discussion, we must keep the task as minimal as possible.
So it comes down to let the loader do that in the `load` function call
as well.

This is also a step towards merging swh.loader.git.base.BaseLoader.

This is a step towards merging swh.loader.git.base.BaseLoader

The initial changelog was not a date.

The debian toolchain must have changed recently and now it results in
the error reported in P97.

Related T495

(Code lost again)

The tree must hold all the needed information (including the children)

(Thus missing the conversion)

swh.loader breaks the pythonpath