Remove unsupported code "discovery_source.go" and increase test coverage

[marckhouzam]

What this PR does / why we need it

The CLI does not support turning off the Central Repository feature. Therefore, the current feature flag constants.FeatureDisableCentralRepositoryForTesting cannot be used, and its unused code paths should be removed.

This PR starts by fixing only the discovery_source.go file and its tests, in an attempt to simplify the review by keeping the changes logically contained.

The changes are:

1. Remove code paths from the discovery_source.go file that only run when the central repo feature is disabled through the use of the feature flag constants.FeatureDisableCentralRepositoryForTesting
2. mark the --uri flag as required for plugin source update; this was forgotten when we implemented the central repo feature (it used to be marked as required); also update the usage text to include the use of the --uri flag since it is now required
3. Update and add more unit tests to increase coverage from 55.5% to 93.7%
4. Fix test for "plugin source list" to use a temporary configuration instead of the user's real CLI config

Which issue(s) this PR fixes

Part of #477

Describe testing done for PR

$ tz version
version: v1.0.0-dev
buildDate: 2023-09-12
sha: 0a6eb1d26

# Check that all the sub-commands are present (except 'delete' which is hidden)
$ tz plugin source -h
Manage plugin discovery sources. Discovery source provides metadata about the list of available plugins, their supported versions and how to download them.

Usage:
  tanzu plugin source [command]

Available Commands:
  init        Initialize the discovery source to its default value
  list        List available discovery sources
  update      Update a discovery source configuration

Flags:
  -h, --help   help for source

Use "tanzu plugin source [command] --help" for more information about a command.

# Check that the 'delete' command is still there
$ tz plugin source delete -h
Delete a discovery source

Usage:
tanzu plugin source delete SOURCE_NAME

Examples:

    # Delete a discovery source
    tanzu plugin discovery delete default

Flags:
  -h, --help   help for delete

# Check the updated help output for "plugin source update"
$ tz plugin source update -h
Update a discovery source configuration

Usage:
tanzu plugin source update SOURCE_NAME --uri <URI>

Examples:

    # Update the discovery source for an air-gapped scenario. The URI must be an OCI image.
    tanzu plugin source update default --uri registry.example.com/tanzu/plugin-inventory:latest

Flags:
  -h, --help         help for update
  -u, --uri string   URI for discovery source. The URI must be of an OCI image

# Check the fact that '--uri' is now marked as required
$ tz plugin source update default
[x] : required flag(s) "uri" not set

# Make sure the command works
$ tz plugin source update default -u projects.registry.vmware.com/tanzu_cli_stage/plugins/plugin-inventory:latest
[i] Reading plugin inventory for "projects.registry.vmware.com/tanzu_cli_stage/plugins/plugin-inventory:latest", this will take a few seconds.
[ok] updated discovery source default
$ tz plugin source list
  NAME                IMAGE
  default             projects.registry.vmware.com/tanzu_cli_stage/plugins/plugin-inventory:latest
  disc_0 (test only)  localhost:9876/tanzu-cli/plugins/airgapped:large
$ tz plugin source init
[ok] successfully initialized discovery source
$ tz plugin source list
  NAME                IMAGE
  default             projects.registry.vmware.com/tanzu_cli/plugins/plugin-inventory:latest
  disc_0 (test only)  localhost:9876/tanzu-cli/plugins/airgapped:large
$ tz plugin source list -o json
[
  {
    "image": "projects.registry.vmware.com/tanzu_cli/plugins/plugin-inventory:latest",
    "name": "default"
  },
  {
    "image": "localhost:9876/tanzu-cli/plugins/airgapped:large",
    "name": "disc_0 (test only)"
  }
]

I also ran the unit tests and checked the test coverage for discovery_source.go: except from minor error case, the file is fully covered.

Release note

Mark the `--uri` flag as required for `tanzu plugin source update`.  Cleanup and increase test coverage for the `tanzu plugin source` sub-commands.

Additional information

Special notes for your reviewer

[anujc25]

LGTM