Enable custom repositories.

[mweiden]

Enables the specification of custom repositories in the transitive_maven_jar function.

For more, see https://github.com/bazelbuild/migration-tooling/issues/14#issuecomment-338764262

This is my first PR to a bazel project, so this will likely need some code review and guidance to reach master. Thanks in advance!

[bazel-io]

Can one of the admins verify this patch?

[googlebot]

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[mweiden]

@google I signed it!

[googlebot]

CLAs look good, thanks!

[wstrange]

@mweiden Nice.

Dumb question: Can this use ~/.m2/settings.xml to authenticate against a private repo such as Artifactory? Is there a way to specify the settings.xml location?

[petroseskinder]

Can this use ~/.m2/settings.xml to authenticate against a private repo such as Artifactory? Is there a way to specify the settings.xml location?

@wstrange. As it stands, this PR allows one to pass urls to maven mirrors as arguments to generate_workspace. Perhaps, we leave settings.xml functionality (#7) to a separate PR.

[mweiden]

@petroseskinder I don't have tests for this. You're right, they should be added. I will try to work on this on Friday.

[mweiden]

@petroseskinder tests added. 👀 please!

[mweiden]

@petroseskinder I've made all but one of the changes you requested, run the code formatter, and updated the jar. Please see my comments regarding the default version resolver in the last outdated diff. I hope that all this looks good and is ready to merge.