[builder] more descriptive empty gomod error

[kristinapathak]

Describe the bug
Providing an empty gomod value in the builder configuration leads to an opaque error.

Steps to reproduce

1. Specify a provider, extension, receiver, exporter, processor, or connector without a gomod value
2. Try to build
3. build fails with error module "": invalid gomod specification for module

What did you expect to see?
An error with a better explanation of where the problem is in the configuration

What did you see instead?

../../bin/ocb_darwin_amd64 --config=./default.yaml
2024-06-27T11:41:54.047-0700	INFO	internal/command.go:125	OpenTelemetry Collector Builder	{"version": "", "date": "unknown"}
2024-06-27T11:41:54.048-0700	INFO	internal/command.go:161	Using config file	{"path": "./default.yaml"}
Error: invalid configuration: module "": invalid gomod specification for module; module "": invalid gomod specification for module

Sample Config

dist:
  module: go.opentelemetry.io/collector/cmd/otelcorecol
  name: otelcorecol
  description: Local OpenTelemetry Collector binary, testing only.
  version: 0.103.0-dev
  otelcol_version: 0.103.0

receivers:
  -
  - gomod: go.opentelemetry.io/collector/receiver/otlpreceiver v0.103.0
providers:
  - gomod: go.opentelemetry.io/collector/confmap/provider/envprovider v0.103.0
  - gomod: go.opentelemetry.io/collector/confmap/provider/fileprovider v0.103.0
  - gomod:
  - gomod: go.opentelemetry.io/collector/confmap/provider/httpsprovider v0.103.0
  - gomod: go.opentelemetry.io/collector/confmap/provider/yamlprovider v0.103.0