feat(prelude): add .dhall extensions on prelude functions

[german1608]

As part of GSoC, I'm currently building a doc generator for the dhall configuration language. One of the issues that I faced was how we should extract files from a dhall package and after several discussions with my mentors (cc: @Gabriel439 , @sjakobi and @Profpatsch) we decided to fetch files by their .dhall extension.

Currently the only files in the dhall prelude that have the .dhall extensions were the package.dhall files in each sub-package and it's really awkard that this tool is not compatible with the main package of dhall, so this PR attempts to add a .dhall extension on each file.

I added them in a compatible way (thanks to @Gabriel439 for the idea): For each file on the package, take the filename and rename it to filename.dhall. Also create a file named filename with the following contents:

./filename.dhall

That way when users refer to, for example, Prelude/Bool/and, dhall will look for the implementation on filename.dhall.

The PR is really big though its a lot of the same

[Gabriella439]

@german1608: You can fix the CI errors by running the ./scripts/lint-prelude.sh script

[german1608]

@Gabriel439 thanks!