planner/core: fix isTableAliasDuplicate, use schema.name as key when table has a alias name

[longfeiss]

What problem does this PR solve?

Issue Number: close #25364

Problem Summary:

What is changed and how it works?

What's Changed:

How it Works:

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Tests

• Manual test (add detailed scripts or steps below)

Side effects

Release note

• No release note

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[xiongjiwei]

hi @longf2021, can you please also take a look at #24622. Many thanks to you.

[longfeiss]

/rebuild

[xuyifangreeneyes]

@oops-lgtm Hi, could you please add test for the pr, thx~

[longfeiss]

close causing it's a wrong solution