[builder] more information for missing gomod error

[kristinapathak]

Description

Fixing a bug - a missing gomod field would be reported but without informing where in the config the field is missing.

Link to tracking issue

Fixes #10474

Testing

Unit tests pass, added some tests for missing cases, manually tested

new error looks like:

../../bin/ocb_darwin_amd64 --config=./default.yaml
2024-06-27T11:39:10.316-0700	INFO	internal/command.go:125	OpenTelemetry Collector Builder	{"version": "", "date": "unknown"}
2024-06-27T11:39:10.319-0700	INFO	internal/command.go:161	Using config file	{"path": "./default.yaml"}
Error: invalid configuration: receiver module at index 0: missing gomod specification for module; provider module at index 2: missing gomod specification for module

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.33%. Comparing base (4a11a3e) to head (ea4a38a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10475   +/-   ##
=======================================
  Coverage   92.33%   92.33%           
=======================================
  Files         393      393           
  Lines       18647    18647           
=======================================
  Hits        17218    17218           
  Misses       1069     1069           
  Partials      360      360           

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.