Add the option to include sources in the native maven_jar rule

[snowp]

This adds the option to download the sources jar (defined as the same maven artifact, but with classifier = "sources") along with the main jar. When doing so it will specify the srcjar attribute on the java_import rule generated by maven_jar.

The motivation behind this is improving IDE integration for third party dependencies, specifically allowing the IJ plugin to automatically attach sources to maven jars without having to use custom Skylark macros.

This also adds in the option to specify a sources_sha1, though I'm not sure how useful that is.

Example of generated BUILD file:

# DO NOT EDIT: automatically generated BUILD.bazel file for maven_jar rule com_google_guava_guava
java_import(
    name = 'jar',
    jars = ['guava-22.0.jar'],
    srcjar = ':sources',
    visibility = ['//visibility:public'],
)

filegroup(
    name = 'file',
    srcs = ['guava-22.0.jar'],
    visibility = ['//visibility:public']
)

filegroup(
    name = 'sources',
    srcs = ['guava-22.0-sources.jar'],
    visibility = ['//visibility:public']
)

Discussed in #1410 & #308

[bazel-io]

Can one of the admins verify this patch?

[cheister]

@juliexxia Can you take a look at this change? It fixes one of the most annoying bugs in the IntelliJ plugin.

[kchodorow]

Thanks for the patch @snowp, but we actually already have a change in progress that's almost ready to merge. It'll basically download the source jar if it's available and put it in the srcjar attribute. Do you need a separate sources target?

[snowp]

@kchodorow Nah I don't have a need for it to be exposed as its own target. Happy to close this PR out in favor of the other change. Is that work being done in the open (i.e. on GitHub)? Would love to follow along.

[juliexxia]

@snowp 7a7c41d should do the trick. LMK if you have any lingering questions/concerns!

[bazel-io]

Can one of the admins verify this patch?