rules_r: retain src dir name in reproducible mode
rules_r: retain src dir name in reproducible mode

Summary:
Some R packages assume some source directory name. And so, all build
modes should use the same directory name.

Ideally, this directory name should be the same as the package name, but
we don't currently guarantee that.

Reviewers: marius

Reviewed By: marius

Maniphest Tasks: T9636

Differential Revision: https://phabricator.grailbio.com/D16415

fbshipit-source-id: 33f5ea6

grailbio/rules_r,grailbio/rules_r,grailbio/rules_r,grailbio/rules_r,grailbio/rules_r