refactor file names, location, and contents

[devoncarew]

• Move all the sdk related code from grinder_tools to grinder_sdk.dart
• rationalize our test dir layout a bit
• add some tests for things in grinder_tools

This is meant to be entirely w/o user impact, just some project hygiene. No changes to code (modulo some new tests).

@seaneagan

[seaneagan]

Removing stuff from grinder_tools.dart is technically a breaking change. Not sure if anyone is actually importing that library, but since it's just in lib rather than lib/src, it's fair game for them to. I'm not sure we need any of the lib/grinder_***.dart libraries (including grinder_sdk.dart in this PR), since we already export them from lib/grinder.dart. If we did keep them I think the grinder_ prefix is redundant.

I personally like to have small libraries under lib/src which import each other as needed. Then just export any public APIs from those in lib/grinder.dart.

[devoncarew]

Removing stuff from grinder_tools.dart is technically a breaking change.

Totally, but my guess is that most people are just importing only grinder.dart, esp. as that exports the other public libs. I'm comfortable with assuming the small change that we break a consumer to get the benefit of rev'ing the major semver fewer times (which has it's own cost downstream).