feat($core): better layout check

[shigma]

Summary

1. layoutComponentMap is generated by readdirSync so there is no need to check existence.
2. If Layout.vue does not exist, Layout.path will always be undefined, which leads to a confusing warning.
3. We don't expect a Layout.vue when a custom GlobalLayout is specified.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

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• Firefox
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If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[flozero]

@shigma can you resolve the conflict please and it will be good for me

[flozero]

maybe update the docs about it

[vue-bot]

Hey @shigma, thank you for your time and effort spent on this PR, contributions like yours help make Vue better for everyone. Cheers! 💚