Repository: SciTools/iris (language: python)

Issue:
standardize requirements structure
## 🚀 Pull Request

### Description
<!-- Provide a clear description about your awesome pull request -->
<!-- Tell us all about your new feature, improvement, or bug fix -->
This PR rationalizes and standardizes the `iris` requirements structure i.e.,

- remove the `requirements/ci` directory
- move the `requirements/ci/nox.lock` directory to `requirements/locks`
- update connective tissue from these changes within `iris` codebase

A subsequent PR will target `SciTools/workflows` to align the `refresh-lockfiles.yml` GHA with this structure.

Reference: 
- https://github.com/SciTools/workflows/pull/8
- https://github.com/SciTools-incubator/iris-esmf-regrid/pull/244
- https://github.com/SciTools/iris-grib/pull/327

---
[Consult Iris pull request check list]( https://scitools-iris.readthedocs.io/en/latest/developers_guide/contributing_pull_request_checklist.html)



Discussion / hints:
## [Codecov](https://codecov.io/gh/SciTools/iris/pull/5204?src=pr&el=h1&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SciTools) Report
Patch and project coverage have no change.
> Comparison is base [(`b228f20`)](https://codecov.io/gh/SciTools/iris/commit/b228f2071e46d940ded4d28fa38b2980e2eafb2d?el=desc&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SciTools) 89.27% compared to head [(`e1436ad`)](https://codecov.io/gh/SciTools/iris/pull/5204?src=pr&el=desc&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SciTools) 89.27%.

<details><summary>Additional details and impacted files</summary>


```diff
@@           Coverage Diff           @@
##             main    #5204   +/-   ##
=======================================
  Coverage   89.27%   89.27%           
=======================================
  Files          88       88           
  Lines       22258    22258           
  Branches     4867     4867           
=======================================
  Hits        19871    19871           
  Misses       1641     1641           
  Partials      746      746           
```



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</details>

[:umbrella: View full report in Codecov by Sentry](https://codecov.io/gh/SciTools/iris/pull/5204?src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SciTools).   
:loudspeaker: Do you have feedback about the report comment? [Let us know in this issue](https://about.codecov.io/codecov-pr-comment-feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SciTools).

@lbdreyer Awesome, thanks :partying_face: 

Fix patch:
diff --git a/.github/workflows/ci-wheels.yml b/.github/workflows/ci-wheels.yml
index 835930e065..843486b84c 100644
--- a/.github/workflows/ci-wheels.yml
+++ b/.github/workflows/ci-wheels.yml
@@ -74,7 +74,7 @@ jobs:
           CACHE_WEEKS: 2
       run: |
         echo "CACHE_PERIOD=$(date +%Y).$(expr $(date +%U) / ${CACHE_WEEKS})" >> ${GITHUB_ENV}
-        echo "LOCK_FILE=requirements/ci/nox.lock/py$(echo ${{ matrix.python-version }} | tr -d '.')-linux-64.lock" >> ${GITHUB_ENV}
+        echo "LOCK_FILE=requirements/locks/py$(echo ${{ matrix.python-version }} | tr -d '.')-linux-64.lock" >> ${GITHUB_ENV}
 
     - name: "conda package cache"
       uses: ./.github/workflows/composite/conda-pkg-cache
@@ -103,7 +103,7 @@ jobs:
     - name: "nox cache"
       uses: ./.github/workflows/composite/nox-cache
       with:
-        cache_build: 0
+        cache_build: 1
         env_name: ${{ env.ENV_NAME }}
         lock_file: ${{ env.LOCK_FILE }}
 


Test patch:
diff --git a/.github/workflows/ci-tests.yml b/.github/workflows/ci-tests.yml
index 6dd79e1c1c..8831329ee3 100644
--- a/.github/workflows/ci-tests.yml
+++ b/.github/workflows/ci-tests.yml
@@ -63,7 +63,7 @@ jobs:
           CACHE_WEEKS: 2
         run: |
           echo "CACHE_PERIOD=$(date +%Y).$(expr $(date +%U) / ${CACHE_WEEKS})" >> ${GITHUB_ENV}
-          echo "LOCK_FILE=requirements/ci/nox.lock/py$(echo ${{ matrix.python-version }} | tr -d '.')-linux-64.lock" >> ${GITHUB_ENV}
+          echo "LOCK_FILE=requirements/locks/py$(echo ${{ matrix.python-version }} | tr -d '.')-linux-64.lock" >> ${GITHUB_ENV}
 
       - name: "data cache"
         uses: ./.github/workflows/composite/iris-data-cache
@@ -111,7 +111,7 @@ jobs:
       - name: "nox cache"
         uses: ./.github/workflows/composite/nox-cache
         with:
-          cache_build: 1
+          cache_build: 2
           env_name: ${{ env.ENV_NAME }}
           lock_file: ${{ env.LOCK_FILE }}