Propagate maven_coordinates tag to generated scala_import so pom generation includes the dependency

[vinnybod]

Description

Add a maven_coordinates= tag to the generated scala maven dependency targets.

Motivation

rules_jvm_external pom generation uses the maven_coordinates=x:y:z tag to determine the formatting of a dependency in the pom. Because this tag is missing, the scala dependencies are excluded from the generated pom.
I need for example @io_bazel_rules_scala_scala_library//jar to appear in my generated pom as

<dependency>
    <groupId>org.scala-lang</groupId>
    <artifactId>scala-library</artifactId>
    <version>2.13.12</version>
</dependency>

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[vinnybod]

Still figuring out who in my company needs to deal with the CLA, but would love some feedback on this in the meantime.

[simuons]

Since coordinates is non mandatory attribute I think tags should be added conditionally rather than having empty maven_coordinates=. wdyt?

[simuons]

Thanks @vinnybod

[liucijus]

@vinnybod to get this merged, CLA needs to be signed

[vinnybod]

@liucijus CLA was signed and submitted yesterday. It was a corporate CLA, so according to opensource.google/documentation/reference/cla#cla-accepted , it might take a couple days for it to process.

[vinnybod]

CLA is approved