Fix packaging warnings for `probe-plugin`

[LukaszKontowski]

In compilation, following warnings appear:

[warn] Warning: {api_2_12} will be merged into non-terminal {probe-plugin_2_12}, other candidates were: Set({pants-probe-plugin_2_12}, {bazel-probe-plugin_2_12}, {scala-probe-plugin_2_12})
[warn] You can specify explicit merge: packageMethod := PackagingMethod.MergeIntoOther(pants-probe-plugin_2_12)
[warn] Warning: {api_2_13} will be merged into non-terminal {probe-plugin_2_13}, other candidates were: Set({pants-probe-plugin_2_13}, {scala-probe-plugin_2_13}, {bazel-probe-plugin_2_13})
[warn] You can specify explicit merge: packageMethod := PackagingMethod.MergeIntoOther(pants-probe-plugin_2_13)
[warn] Warning: {api_2_12} will be merged into non-terminal {probe-plugin_2_12}, other candidates were: Set({pants-probe-plugin_2_12}, {bazel-probe-plugin_2_12}, {scala-probe-plugin_2_12})
[warn] You can specify explicit merge: packageMethod := PackagingMethod.MergeIntoOther(pants-probe-plugin_2_12)
[warn] Warning: {api_2_13} will be merged into non-terminal {probe-plugin_2_13}, other candidates were: Set({pants-probe-plugin_2_13}, {scala-probe-plugin_2_13}, {bazel-probe-plugin_2_13})
[warn] You can specify explicit merge: packageMethod := PackagingMethod.MergeIntoOther(pants-probe-plugin_2_13)

The reason of this issue is to fix this.

[PawelLipski]

-Xfatal-warnings to be added to flags once this is fixed

[LukaszKontowski]

These warnings come from the sbt-idea-plugin:
https://github.com/JetBrains/sbt-idea-plugin/blob/master/packaging/src/main/scala/org/jetbrains/sbtidea/packaging/mappings/LinearMappingsBuilder.scala#L86-L87

[LukaszKontowski]

Waiting for sbt-idea-plugin team to respond: JetBrains/sbt-idea-plugin#118

[PawelLipski]

So to recap... -Xfatal-warnings can be added regardless? let's assume the answer on the issue in sbt-idea-plugin won't come anytime soon

[LukaszKontowski]

Yes - PR created #247

[PawelLipski]

Unassigning Łukasz as this is blocked on JetBrains, dunno when they're gonna respond :/