Remove src/__init__.py and fix many errors

[jdahm]

Description

Major changes:

• Adds type checking on a few more files.
• Expr.dtype's type is no longer Optional[DataType], because that caused type havoc. Therefore, the default is changed from None to DataType.AUTO when not set explicitly. The gtir_dtype_resolver and other visitors are updated to check for DataType.AUTO instead of None.
• Moves most of the code that was in eve.iterators to eve.concepts to work around a circular dependency after solving ClassVar parameters cannot include any type variables #565 using @egparedes's suggestion. This is a temporary workaround until we replace Eve with the new version.
• gt4py.stencil_object.ArgsInfo is made non-private and used in gt4py.dace_stencil_object to fix a type issue.
• Ignores typing in eve visitor contexts -- need to fix that in a follow-up PR.
• npir.HorizontalMask is created as a tuple of tuples, to avoid the Optional[AxisBound] in the common.HorizontalInterval type.
• Replaces gt_backend references with gt4py.backend.

Follows #978.

Resolves #565.
Resolves #992.
Resolves #1007.

[gronerl]

Looks like there's a circular import problem now, how should we proceed with this?

[jdahm]

Looks like there's a circular import problem now

I committed the code and pushed after nearly a whole day of working out these issues, only to find out about it 😆 .

how should we proceed with this?

The circular dependency should be solved, or rather avoided, by moving to the next version of eve. But in this case a bit of analysis and moving around code definitions within this version of eve could solve it.

[jdahm]

bors try

[bors]

try

Build failed:

• ci/gitlab/full-pipeline

[jdahm]

bors try

[bors]

try

Build failed:

• ci/gitlab/full-pipeline

[jdahm]

bors try

[bors]

try

Timed out.

[egparedes]

It's impressive the amount of work you've done in this PR to fix all typings! It looks good to me, just a couple of very minor questions and suggestions, but I don't see anything blocking the merge.

[egparedes]

It looks good to go.

[jdahm]

bors try

[bors]

try

Build succeeded:

• ci/gitlab/full-pipeline