Reorganize lib/Conversion the same way as tests

[AlexanderViand-Intel]

Following the test reorganization, this does the same for /lib: Conversion/SomedialectToOtherdialect is moved to Dialect/Somedialect/Conversions/SomedialectToOtherdialect.

EDIT: I've also created a /lib/Utils/ folder where I put Conversion/Utils.{h/cpp}, and the Graph and DRR helpers

@ai-mannamalai I've not adapted the CMake build in this one, as my next PR should be the full CMake-build-with-dependencies :)

Open ToDos:

• [ ] Fix Docs/Website generation
• [ ] Update template scripts

Depends on

• #1019