[steps] fix resolving custom function path

[szdziedzic]

Why

The relative function path was resolved always based on a dirname of the main config path, not based on dirname of the imported config file which actually contains the function declaration.

How

Resolve relative function path based on the path to the config file which actually contains the function

Test Plan

Tests

[codecov]

Codecov Report

Merging #259 (d1a4aec) into main (21dfc76) will decrease coverage by 0.17%.
The diff coverage is 84.22%.

@@            Coverage Diff             @@
##             main     #259      +/-   ##
==========================================
- Coverage   93.24%   93.06%   -0.17%     
==========================================
  Files          21       21              
  Lines         843      850       +7     
  Branches      179      181       +2     
==========================================
+ Hits          786      791       +5     
- Misses         57       59       +2     

Files Changed	Coverage Δ
packages/steps/src/cli/cli.ts	0.00% <0.00%> (ø)
packages/steps/src/BuildConfig.ts	93.21% <84.62%> (-1.36%)	⬇️
packages/steps/src/BuildConfigParser.ts	100.00% <100.00%> (ø)
packages/steps/src/BuildStepContext.ts	89.10% <100.00%> (-0.19%)	⬇️