Fix source initialization order

[adriansmares]

Summary

This PR fixes the fact that source commands are created after the sources are registered. This causes the flag set of the source to be empty.

Before:

go run ./cmd/ttn-lw-migrate chirpstack devices --help
Export devices by DevEUI

Usage:
  ttn-lw-migrate chirpstack device ... [flags]

Aliases:
  device, end-devices, end-device, devices, devs, dev, d

Flags:
  -h, --help   help for device

Global Flags:
      --dry-run                      Do everything except resetting root and session keys of exported devices
      --frequency-plans-url string   URL for fetching frequency plans (default "https://raw.githubusercontent.com/TheThingsNetwork/lorawan-frequency-plans/master")
      --verbose                      Verbose output

After:

go run ./cmd/ttn-lw-migrate chirpstack devices --help
Export devices by DevEUI

Usage:
  ttn-lw-migrate chirpstack device ... [flags]

Aliases:
  device, end-devices, end-device, devices, devs, dev, d

Flags:
      --api-ca string              (optional) CA for TLS
      --api-insecure               Do not connect to ChirpStack over TLS
      --api-token string           ChirpStack API Token
      --api-url string             ChirpStack API URL
      --export-session             Export device session keys from ChirpStack (default true)
      --export-vars                Export device variables from ChirpStack
      --frequency-plan-id string   Frequency Plan ID of exported devices
  -h, --help                       help for device
      --join-eui string            JoinEUI of exported devices

Global Flags:
      --dry-run                      Do everything except resetting root and session keys of exported devices
      --frequency-plans-url string   URL for fetching frequency plans (default "https://raw.githubusercontent.com/TheThingsNetwork/lorawan-frequency-plans/master")
      --verbose                      Verbose output

Changes

• Register the sources next to the source commands.

Testing

Local testing - see above.

Regressions

N/A. This fixes a regression.

Notes for Reviewers

@KrishnaIyer please take a look.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Documentation: Relevant documentation is added or updated.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.

[KrishnaIyer]

Damn