different argument name for sha256 in jvm_import_external rule in bazel repo and this repo

`jvm_import_external` has duplicate definitions with different arg name for the same field:
in [bazel repo](https://github.com/bazelbuild/bazel/blob/master/tools/build_defs/repo/jvm.bzl#L211) the arg name is `artifact_sha256`
in [rules_scala](https://github.com/bazelbuild/rules_scala/blob/6a9f81aa29563a07cc69a2555e54ac3cdfd396ed/scala/scala_maven_import_external.bzl#L211) the arg name is `jar_sha256`

I think rules_scala should not have an implementation of jvm_import_external at all...
so this will probably mean adding an alias to bazel repos's definition...

Compare the definitions at tools/build_defs/repo/jvm.bzl around line 211 and scala/scala_maven_import_external.bzl around line 211, then inspect how rules_scala callers use jvm_import_external. Done means the duplicate implementation is no longer inconsistent with Bazel's definition and the SHA-256 argument has one compatible name or alias.

Written by the indexing model from the issue text.