feat(manifests) resource kind is case-insensitive (internal refactor)

[dduportal]

Depends on #388 , #414, #415, #418, #453, #456, #457, #479, #480, #481, #482 (will be rebased once all merged).

This PR is a refactor around the plugins to factorize common behaviors of resources (e.g. sources, conditions and targets) when possible, by using struct composition (https://golangbot.com/inheritance/) to look-a-like OO Object-Oriented's inheritance.

The expected outcomes are:

• (end-users) the type of resource, specified by the attribute kind, is now case un-sensitive
• (contributors) A single place to initialize the list of plugins (centralized unmarshaling for instance)
• (contributors) improved code convention for plugins (meaning constant behavior)
• (contributors) code base easier to browse as resources plugins are separated from scms plugins and "utils/generic" plugins

I recommend you to review this PR commit by commit (without checking the 4 commits from #482 of course) since it is refactoring!
It introduces the following changes (1 per commit):

• Introduces a new package resource with a ResourceConfig struct. This package defines the common behavior that any plugin resource should implement.
• Factorize the unmarshalling away from source, condition and target, into this new resource package. It allows a centralized place to list the resource plugin + delegating parsing logic on each resource plugin.
• This "centralization" allows to define the kind of each resource to be case insensitive 🥳
• The changelog retrieval is now implemented by all the resources. If not implemented, then it returns an empty string. It makes the changelog retrieval way easier to implement/change and makes it usable with the internal state of resources.
• Refactor the directory + package structure of plugin. It makes it really clear which package implements which kind of "behavior" in updatecli

$ tree -L 1 pkg/plugins
pkg/plugins
├── resources # plugins that implements the interface resource (e.g. a source/condition/target)
├── scms # plugins that implements the SCM interface (git / github)
└── utils # other plugins which behaviors are reused across other plugins but not in core

Test

To test this pull request, you can run the following commands:

make test
make test-short
make test-e2e

Additional Information

Please note that this PR lays the foundation for #373, #413 and #497 (deprecating unused fileds)

[olblak]

@dduportal Well done, it's a nice pull request. It's interesting to see that you implemented in ways hesitated to do. I hesitated a Like the same interface for source, condition, target, but in the end, your approach makes things clearer.
Considering that it's a very big refactoring, I would like to take additional time for testing.

[olblak]

I think before merging this PR we should also clean up the CONTRIBUTING

[olblak]

So far I didn't identify any regression 🎉