Integrate validate_spec with combine_specs

[kenodegard]

Since validations are equally important after combining specs, perform the validations while combining in order to alert the user as to which spec is causing the issue for more helpful debugging. Otherwise sought to standardize terminology (e.g. use explode instead of expand).

Changes:

• ~~added include_dict argument to utils.islist to control whether a dict is considered a list or not~~ (see #4088)
• ~~added some terminology definitions at the top of variants, hopefully, this will make variants less daunting to understand by future devs (read me)~~ (see #4086)
• variants.validate_spec was combined with variants.combine_specs as I found that combining necessitates similar checks being done, as a backwards compatible solution variants.validate_spec is now just a call to variants.combine_specs with a single spec
• ~~a slight update to variants.find_config_files inorder to make the call from variants.combine_specs cleaner~~ (see #4086)

[kenodegard]

@mingwandroid phew it has taken me a while to get back here (yay for other projects consuming all of my time...), I'm not necessarily in love with this implementation (mostly due to the high code complexity) so I would like some feedback on whether this is a good direction

[github-actions[bot]]

Hi there, thank you for your contribution!

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed automatically if no further activity occurs.

If you would like this pull request to remain open please:

1. Rebase and verify the changes still work
2. Leave a comment with the current status

NOTE: If this pull request was closed prematurely, please leave a comment.

Thanks!

[github-actions[bot]]

Hi there, thank you for your contribution!

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed automatically if no further activity occurs.

If you would like this pull request to remain open please:

1. Rebase and verify the changes still work
2. Leave a comment with the current status

NOTE: If this pull request was closed prematurely, please leave a comment.

Thanks!