Reject mismatched sources rules changes

[mstemm]

What type of PR is this?

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/kind bug

/kind cleanup

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/kind failing-test

/kind feature

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Any specific area of the project related to this PR?

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What this PR does / why we need it: This fixes a bug where a second rule definition for a different source is mistakenly combined with the first rule definition of a different source. This now returns the same error as when defining a second rule.

Which issue(s) this PR fixes:

Fixes https://github.com/falcosecurity/falco/issues/3382

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Explicitly disallow appending/modifying a rule with different sources.