Adopting directives_ordering and prefer_relative_imports lints

[domesticmouse]

Based on #57

[esDotDev]

Curious, is prefer_relative_imports now recommended by the flutter team? I know that mixing relative and package can lead to confusion, so we had standardized on package imports.

From my experience, Android Studio can often fail to provide code-hinting when trying to import things using relative import, and refactoring is also more likely to break when you drag a file into a different folder, which is why we ended up settling on package level.

For example, IDE will not return a relative import for me here for some reason:

[domesticmouse]

Curious, is prefer_relative_imports now recommended by the flutter team?

If you look at the state before https://github.com/gskinnerTeam/flutter-folio/pull/57/files you will see that the analyzer did contain prefer-relative-imports: true which wasn't enfoced because it was mis-spelt.

I personally like prefer_relative_imports because it tells me whether it is an in-project reference, or an external dependency. I tie this with directives_ordering to keep everything organised.

As to the IDE returning the wrong suggestion, I'd raise that as a bug against the Dart Tools team, as it's their code that is returning that suggestion. However, I suspect if you accepted the suggestion, the following cleanup suggestion from the tools would convert it into a local path. I didn't hand convert all the imports to relative paths, I just clicked the drop down in VS Code to organise the imports.

If this change isn't right for you, then don't merge this PR =)