Allow import paths to be workspace-relative

[alexeagle]

In https://github.com/bazelbuild/rules_typescript/issues/77 we let you import {} from 'my_wksp/src/some_file'

but in some existing codebases there's an assumption that TypeScript is configured such that you can

import {} from 'src/some_file'

rather than require these applications to change all their imports, maybe we could allow the ts_library rule to be configured to work.