feat(sbt): support anonymous objects and dotted symbols for version declaration

[Thooms]

Changes

Support anonymous object version definition in the sbt plugin.

Context

The following is a commonly used pattern:

val versions = new {
  example = "0.0.8"
}
libraryDependencies += "org.example" % "foo" % versions.example

This PR brings support to this, only on one level of nesting.

Please select one of the following:

• This closes an existing Issue, Closes: #
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

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Did you use AI tools to create any part of this pull request?

Please select one option and, if yes, briefly describe how AI was used (e.g., code, tests, docs) and which tool(s) you used.

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• Yes — substantive assistance (AI-generated non‑trivial portions of code, tests, or documentation).
• Yes — other (please describe):

Documentation (please check one with an [x])

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I have verified these changes via:

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

The public repository: https://github.com/Thooms/renovate-example-scala-repository

Example of a created PR: Thooms/renovate-example-scala-repository#4