Access DEBUG flag through get_flags()

[Threynaud]

Resolves #11068

Problem

ManifestLoader.check_for_spaces_in_resource_names() causes an AttributeError in certain cases, for instance when used with SQLFluff dbt templater.
AttributeError: 'DbtConfigArgs' object has no attribute 'DEBUG'

Solution

Access the DEBUG flag through the get_flags()method as done everywhere else in the codebase.

Checklist

• [ x] I have read the contributing guide and understand what's expected of me.
• [ x] I have run this code in development, and it appears to resolve the stated issue.
• [x ] This PR includes tests, or tests are not required or relevant for this PR.
• [ x] This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• [ x] This PR includes type annotations for new and modified functions.

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[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: Thomas Reynaud.
This is most likely caused by a git client misconfiguration; please make sure to:

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[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: Thomas Reynaud.
This is most likely caused by a git client misconfiguration; please make sure to:

1. check if your git client is configured with an email to sign commits git config --list | grep email
2. If not, set it up using git config --global user.email [email protected]
3. Make sure that the git commit email is configured in your GitHub account settings, see https://github.com/settings/emails