fix(manager/gradle): prefix version catalog aliases with "libs."

[Churro]

Changes

Prefix version catalog aliases with libs, as the version catalog default name, to prevent confusion with variables that have the same name and to comply with gradle behavior:

For example the aliases groovyCore, groovyJson and groovyXml would be mapped to the libs.groovyCore, libs.groovyJson and libs.groovyXml accessors respectively.

This also fixes a related issue: renovate stores library aliases as variable for later use. Since gradle configs reference them with the catalog name prefixed only, these references never worked until now, since the prefix wasn't prepended.

Context

Closes #18825

Documentation (please check one with an [x])

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How I've tested my work (please tick one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

Test repo: renovate-demo/18825#1

[renovate-release]

🎉 This PR is included in version 34.21.3 🎉

The release is available on:

• GitHub release
• 34.21.3

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