Make the settings attribute of Skylark maven_jar and maven_aar a label.

Fixes https://github.com/bazelbuild/bazel/issues/2117.

The settings attribute allows you to optional specify a custom Maven
settings.xml file as a label. Previously, this attribute was an absolute path.
As an absolute path it cannot really be used to specify a file in the workspace
because each developer may install the workspace in a different location.
And if the settings.xml cannot be included in the workspace, the developers may
as well use one of Maven's default locations for settings.xml.

Now the attribute is a label. An example use case is:

    $ cat WORKSPACE
    load("@bazel_tools//tools/build_defs/repo:maven_rules.bzl")
    maven_jar(
        name = "guava",
        artifact = "com.google.guava:guava:19.0",
        settings = "//:my_custom_settings.xml",
    )
    $ cat BUILD
    java_library(
        srcs = glob(["**/*.java"]),
        deps = ["@guava//jar"],
    )
    $ cat my_custom_settings.xml
    <settings xmlns="http://maven.apache.org/SETTINGS/1.0.0
                     https://maven.apache.org/xsd/settings-1.0.0.xsd">;
      <mirrors>
        <mirror>
          <id>planetmirror.com</id>
          <name>PlanetMirror Australia</name>
          <url>http://downloads.planetmirror.com/pub/maven2</url>;
          <mirrorOf>central</mirrorOf>
        </mirror>
      </mirrors>
    </settings>

Note that `settings = "my_custom_settings.xml"` will not work as the workspace
rule will not know to look in the main workspace.

RELNOTES[INC]: Skylark maven_jar and maven_aar settings attribute is now a label so
it can be checked into your workspace.

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PiperOrigin-RevId: 140861633
MOS_MIGRATED_REVID=140861633

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