FR: generate linkable alternative target for neverlink jars (reopen)

Following issue #185

We actually have the same issue at Wix and we would love to have the same some built in solution for linkable target. We prefer not to have multiple maven_install calls as it generates two different dependency trees.

How about generating a linkable target under `@maven//linkable:`? e.g: `@maven//linkable:javax_servlet_javax_servlet_api`

The content of it would be similar to the BUILD file in the root of the generated external repository but the rules would not have `neverlink=True`?

We would love to implement it, if we agree that it's a good solution.

cc: @liucijus @ittaiz @natansil , @jin

Start by reading issue #185 and the generated external repository's root BUILD file, both referenced in the issue. Determine how a @maven//linkable: target should be generated without neverlink=True while preserving one dependency tree; done means the proposed linkable targets are available for the requested artifacts and the existing targets remain intact.

Written by the indexing model from the issue text.