Refactor project lookup to return bool instead of sentinel error

[begelundmuller]

• Fixes a place where the error got rewritten, leading to accidental Warning: failed to pull environment credentials: in rill validate
• Changes ErrNoMatchingProject to ErrInferProjectFailed to make it less confusing (it's not clear to the user that inference is happening, so "matching" the previous error didn't accurately convey what went wrong)
• Removes a reference to the cli package from the runtime

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!