add explicit type

[xuwei-k]

prepare Scala 2.13.12

• Emit migration warnings under -Xsource:3 as fatal warnings, not errors; -Xmigration turns off erroring scala/scala#10439
• https://github.com/scala/community-build/blob/12866dc60bf80ea569f8a7e4d229a8afe04d84a1/proj/mima.conf#L16-L18

[error] core/src/main/scala/com/typesafe/tools/mima/core/PackageInfo.scala:14:7: under -Xsource:3, inferred com.typesafe.tools.mima.core.PackageInfo instead of com.typesafe.tools.mima.core.NoPackageInfo.type [quickfixable]
[error] Scala 3 migration messages are errors under -Xsource:3. Use -Wconf / @nowarn to filter them or add -Xmigration to demote them to warnings.
[error] Applicable -Wconf / @nowarn filters for this fatal warning: msg=<part of the message>, cat=scala3-migration, site=com.typesafe.tools.mima.core.NoPackageInfo.owner
[error]   val owner       = this
[error]       ^
[error] core/src/main/scala/com/typesafe/tools/mima/core/PackageInfo.scala:15:7: under -Xsource:3, inferred com.typesafe.tools.mima.core.Definitions instead of Nothing [quickfixable]
[error] Scala 3 migration messages are errors under -Xsource:3. Use -Wconf / @nowarn to filter them or add -Xmigration to demote them to warnings.
[error] Applicable -Wconf / @nowarn filters for this fatal warning: msg=<part of the message>, cat=scala3-migration, site=com.typesafe.tools.mima.core.NoPackageInfo.definitions
[error]   def definitions = sys.error("Called definitions on NoPackageInfo")
[error]       ^

[lightbend-cla-validator]

Hi @xuwei-k,

Thank you for your contribution! We really value the time you've taken to put this together.

We see that you have signed the Lightbend Contributors License Agreement before, however, the CLA has changed since you last signed it.
Please review the new CLA and sign it before we proceed with reviewing this pull request:

https://www.lightbend.com/contribute/cla