[build-tools] Load legacy command/path local functions and resolve module paths

[sswrk]

Why

Build runners load referenced local functions from the checked-out project sources before parsing the job, so they need to understand the single-step shape added in #4095/#4096. A path function also needs its module directory resolved on the runner.

How

Parses function.yml with the widened LocalFunctionConfigZ and returns either shape.

For a path function, the module path is resolved against the function's own directory, mirroring how a .eas/build config resolves it against the config file, using the shared resolveLegacyFunctionModulePath helper. The loader fails early with a clear error naming both the literal path: value and the resolved location when the module directory or its package.json is missing. This check has to live here: single-step functions never join workflow.buildFunctions, so BuildWorkflowValidator.validateCustomFunctionModulesAsync will not see them.

The absolute path stays under projectSourceDirectory, so the existing source-to-target remap in createCustomFunctionCall keeps working exactly as it does for .eas/build functions. buildCompositeFunctionCatalogAsync and its callers (generic.ts, jobHooks.ts, custom.ts) need no changes; they delegate to the shared catalog builder.

Test Plan

cd packages/build-tools && yarn test src/steps/__tests__/compositeFunctions.test.ts

Loader tests: a command function loads with its inputs, outputs, shell and platforms; a path function's module path is rewritten to an absolute path under the function directory; a missing module directory and a module without package.json both fail fast; a catalog mixing composite and single-step functions loads; a config mixing both shapes reports the prettified error.

Also smoke-tested end to end against a scratch project: a command function with an input ran and set its output through set-output, and a path function using the prebuilt my-custom-ts-function fixture ran and set its outputs.

[linear-code]

ENG-25402

[sswrk]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• [eas-cli] Validate legacy command/path local functions referenced from workflows #4098
• [build-tools] Load legacy command/path local functions and resolve module paths #4097 👈 (View in Graphite)
• [steps] Expand legacy command/path local functions to a single build step #4096
• [eas-build-job] Accept legacy command/path custom function shape in function.yml #4095
• [eas-cli] Validate local composite functions referenced from workflow hooks #4064
• [build-tools] Load composite function catalogs for hook steps #4063
• [steps] Support local composite functions in hook steps #4062
• [steps] Unify the default gating of steps with no if: across steps, composite scopes, and hooks #4061
• [eas-cli] Validate local composite functions in workflow files #3931
• [build-tools] Load local composite function catalog at build runtime #3930
• [steps][4/4] Resolve composite function outputs #4015
• [steps][3/4] Interpolate composite function inputs #4014
• [steps][2/4] Expand composite function references into build steps #3929 : 1 other dependent PR (#4013 )
• [steps][1/4] Add isCompositeFunctionPath and reject local composite functions in custom builds #3956
• [eas-build-job] Add local composite function schema #3928
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.33%. Comparing base (096a5fd) to head (548b648).

Additional details and impacted files

@@                                   Coverage Diff                                    @@
##           szymonswierk/eng-25402-steps-expand-legacy-functions    #4097      +/-   ##
========================================================================================
+ Coverage                                                 62.32%   62.33%   +0.02%     
========================================================================================
  Files                                                       996      996              
  Lines                                                     44867    44881      +14     
  Branches                                                   9440     9443       +3     
========================================================================================
+ Hits                                                      27959    27973      +14     
  Misses                                                    15462    15462              
  Partials                                                   1446     1446              

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[github-actions]

✅ Thank you for adding the changelog entry!