This drops the pytest-postgresql from requirements-db.txt. This is not a
required dependency for the swh.core.db module itself.

It's a requirement for the swh.core.db.pytest_plugin. This module should become
its own module as a test requirements for other swh modules (including
swh.core.db but not limited to it).

So this commit adds a dedicated requirements-db-pytestplugin.txt file with such
dependency. This opens the db-pytestplugin key within the setup.py and then
references that test dependency for the swh.core.db modules (as it's needed for
tests of the module).

Related to T2746

the idea is now to be a bit more explicit, and expect thoses commands
to have the main `swh` click group as parent.

Then the swh.cli.subcommands entrypoint hooks only have to load the
declared cli modules.

So we now expect to:

- use swh.core.cli.swh as parent group when declaring
  sub-groups/commands in a swh package.
- the swh.cli.subcommand entrypoint to only refer the module rather than
  the click group/command, ie. have:

  '''
    [swh.cli.subcommands]
    name = swh.path.to.cli_module
  '''

  instead of:

  '''
    [swh.cli.subcommands]
    name = swh.path.to.cli_module:click_command
  '''

A backward compatilibility mech is provided: if the loaded entrypoint
object is indeed a click command, auto add it to the main swh group.

Related to T2610

Related to T2105

Related to T2367

I forgot to run it on files at the root in the previous commit.

Roughly, the idea is that http queries are transformed into file queries within
http context. If found, the data file is served or not (404). This translates
into query response so that the code can actually complete as usual.

For technical reasons the pytest plugin is in a dedicated non-swh module. This
is actually required for the pytest plugin loading mecanism to work without
hijacking the loading of swh packages. See [1] for more details.

[1] https://github.com/pytest-dev/pytest/issues/2042

that allows to initialize the database for a given swh package with db
connection credentials/dsn read from a standard swh config file.

Also add a couple of tests (only tests click group and command exist).

Closes T1896.

Note that for now, the main requirements.txt still contains all the
dependencies for backward compatibility reasons.
This will have to be fixed once all packages that depends on swh.core
have been updated.

closes T1678.

This allows to declare click-based subcommands using the setuptools'
entry point "swh.cli.subcommands".

The main "swh" command group does only handle the "--log-level" option.

Also extract the db-init command in a dedicated submodule and ensure both
  swh db-init
and
  swh-db-init
work.

Related to T1671.

used to initialise a database for a given (swh) module, like:

  swh-db-init storage

- add classifiers informations,
- add the content of the README file as long_description,
- add configure_urls.

related to T1242