Test whether duplicate go modules break the builder

[jpkrohling]

Some go modules might contain multiple components. In the builder, each component would be an entry in the manifest, which would cause it to add the same entry multiple times to the go.mod file. Need to verify that cherry-picking individual components works, and come up with a fix in case it doesn't.

[jpkrohling]

This manifest does work:

dist:
  module: github.com/open-telemetry/opentelemetry-collector-builder/test/nocore
  otelcol_version: 0.21.0
  include_core: false

receivers:
  - import: go.opentelemetry.io/collector/receiver/jaegerreceiver
    gomod: go.opentelemetry.io/collector v0.21.0
exporters:
  - import: go.opentelemetry.io/collector/exporter/loggingexporter
    gomod: go.opentelemetry.io/collector v0.21.0
extensions:
  - import: go.opentelemetry.io/collector/extension/healthcheckextension
    gomod: go.opentelemetry.io/collector v0.21.0

We might want to remove the special code around the Core property from the Module type, as it's not really necessary, given the example above.