refactor: squash modules into one to enable optimizations

[kszucs]

At this point the benchmarks show impressive results:

koerce/tests/test_y.py::test_pydantic PASSED
koerce/tests/test_y.py::test_annotated PASSED


-------------------------------------------------------------------------------------------- benchmark: 2 tests -------------------------------------------------------------------------------------------
Name (time in ns)            Min                     Max                  Mean              StdDev                Median                IQR            Outliers  OPS (Kops/s)            Rounds  Iterations
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
test_annotated          512.7001 (1.0)        3,164.3000 (1.0)        596.8280 (1.0)      148.8698 (1.0)        561.4397 (1.0)      38.2401 (1.0)      802;2463    1,675.5245 (1.0)       20000          50
test_pydantic         1,223.6397 (2.39)     105,432.3203 (33.32)    1,447.6836 (2.43)     978.5593 (6.57)     1,318.7603 (2.35)     95.6000 (2.50)     806;2200      690.7587 (0.41)      20000          50
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

[kszucs]

I wasn't able to cimport from pure python cythonized modules even with agumenting pxd files, so I started to move over to pure cython syntax, but the development time has increased which made me value fast iterations during development even more.
So I had to come up with a solution so that the implementation is being split into modules (easier maintanence) but still profit from the timing. The solution was to squash all the python files into a single one before cythonizing, in order to make it work the internal imports must be removed.

[cpcloud]

I guess we'll see whether this becomes a burden long term. For now, the solution seems good!

[kszucs]

There is already a feature request on cython's side, hopefully this will be fixed in the meantime cython/cython#4892

[cpcloud]

Seems legit, if slightly annoying that this is required.