[9.x.x] Improved EditorResourceAsset loading and shader nullreference exceptions

[ellioman]

Purpose of this PR

This PR solves two issues:

• Performance issues when opening a big project in URP due to a call to AssetDatabase.FindAssets(). It is now replaced with a LoadAssetAtPath using the asset's GUID.
• NullReferenceExceptions when trying to load several shaders when building a project in URP. We now do the same as HDRP, which is to return null if the editor resource is not available.

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Checklist for PR maker

• Have you added a backport label (if needed)? For example, the need-backport-* label. After you backport the PR, the label changes to backported-*.
• Have you updated the changelog? Each package has a CHANGELOG.md file.
• Have you updated or added the documentation for your PR? When you add a new feature, change a property name, or change the behavior of a feature, it's best practice to include related documentation changes in the same PR.
• Have you added a graphic test for your PR (if needed)? When you add a new feature, or discover a bug that tests don't cover, please add a graphic test.

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Testing status

Manual Tests:

• Opened test project + Run graphic tests locally
• Built a player
• Checked new UI names with UX convention
• Tested UI multi-edition + Undo/Redo + Prefab overrides + Alignment in Preset
• C# and shader warnings (supress shader cache to see them)
• Checked new resources path for the reloader (in developer mode, you have a button at end of resources that check the paths)
• Other:

Automated Tests:
Added a check in the ValidateBuiltinResourceFiles test where we now also test that the editor resource asset is loaded.

Yamato:
https://yamato.prd.cds.internal.unity3d.com/jobs/902-Graphics/tree/universal%252Feditorresource-and-speedtree-fixes

[erikabar]

Tested Boat Attack models, also grabbed more models from the Asset Store - everything is now importing faster. NullReferenceExceptions are gone.