Exclude development tools from unused dependencies detection

[zz1874]

Description

Closes #117

This PR addresses the issue of handling tools (development dependencies) that are reported as "unused" in the project's dependencies. The solution involves introducing a default_ignore_unused list that includes common development tools known not to be imported directly in the project's scripts.

There are two scenarios:

• If the user does not provide their own ignore_unused option, Fawltydeps will use this default_ignore_unused list as its default, automatically detecting and ignoring the specified development tools.
• If the user provides their own ignore_unused option, it will overwrite the default one, and Fawltydeps will use the user-defined list instead.

Changes Made

• Added a default_ignore_unused list to specify default development tools to be ignored.
• Updated unit tests test_cmdline_on_ignored_undeclared_option to include check_unused_action_on_default_ignored_unused_dep__outputs_nothing (automatically detect using default ignore_unused list) and check_unused_action_on_overriden_ignored_unused_dep__outputs_nothing (using user defined ignore_unused list)
• Updated integration tests for real projects

[jherland]

This looks really good, except for a couple of minor changes outlined below (most of my comments are just copies of one change).

In addition to this, I'd like to see an update to the documentation of the --ignore-unused option, to reflect the new default/behavior.

I'm not sure if we should also update the help text:

$ fawltydeps --help
...
  --ignore-unused DEP_NAME [DEP_NAME ...]
                        Dependencies to ignore when looking for unused dependencies, e.g. --ignore-unused pylint black
...

[jherland]

No further objections, your honor. 😉

Great work! 🚀