refactor: rename old_codegen -> codegen

[charles-cooper]

What I did

Pretty straightforward, also cleaned up some import statements

How I did it

How to verify it

Description for the changelog

Cute Animal Picture

[codecov-commenter]

Codecov Report

Merging #2595 (0115366) into master (ef1e6e4) will decrease coverage by 0.00%.
The diff coverage is 97.45%.

@@            Coverage Diff             @@
##           master    #2595      +/-   ##
==========================================
- Coverage   86.32%   86.31%   -0.01%     
==========================================
  Files          90       90              
  Lines        9291     9290       -1     
  Branches     2353     2353              
==========================================
- Hits         8020     8019       -1     
  Misses        789      789              
  Partials      482      482              

Impacted Files	Coverage Δ
vyper/cli/vyper_lll.py	0.00% <0.00%> (ø)
vyper/cli/vyper_serve.py	0.00% <0.00%> (ø)
vyper/codegen/function_definitions/__init__.py	100.00% <ø> (ø)
vyper/codegen/function_definitions/utils.py	100.00% <ø> (ø)
vyper/codegen/memory_allocator.py	64.51% <ø> (ø)
vyper/codegen/types/__init__.py	100.00% <ø> (ø)
vyper/codegen/types/types.py	83.33% <ø> (ø)
vyper/cli/vyper_compile.py	74.16% <50.00%> (ø)
vyper/ast/signatures/function_signature.py	87.70% <100.00%> (ø)
vyper/ast/signatures/interface.py	81.48% <100.00%> (ø)
... and 26 more

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[lgtm-com]

This pull request introduces 1 alert and fixes 1 when merging 9180758 into ef1e6e4 - view on LGTM.com

new alerts:

• 1 for 'import *' may pollute namespace

fixed alerts:

• 1 for 'import *' may pollute namespace

[lgtm-com]

This pull request introduces 1 alert and fixes 1 when merging 0115366 into ef1e6e4 - view on LGTM.com

new alerts:

• 1 for 'import *' may pollute namespace

fixed alerts:

• 1 for 'import *' may pollute namespace