Alias module deduping

[guybedford]

Feature Use Case

When defining multiple alias modules as entry points, where those alias modules alias a module that is not an entry point, the aliased module is still output as a separate chunk. See the example at http://rollupjs.org/repl/?version=2.8.2&shareable=JTdCJTIybW9kdWxlcyUyMiUzQSU1QiU3QiUyMm5hbWUlMjIlM0ElMjJtYWluLmpzJTIyJTJDJTIyY29kZSUyMiUzQSUyMmltcG9ydCUyMHBqc29uJTIwZnJvbSUyMCcuJTJGcGFja2FnZS5qc29uJyUzQiU1Q25jb25zb2xlLmxvZyhwanNvbiklM0IlMjIlMkMlMjJpc0VudHJ5JTIyJTNBdHJ1ZSU3RCUyQyU3QiUyMm5hbWUlMjIlM0ElMjJwYWNrYWdlLmpzb24lMjIlMkMlMjJjb2RlJTIyJTNBJTIyZXhwb3J0JTIwJTdCJTIwZGVmYXVsdCUyMCU3RCUyMGZyb20lMjAnLiUyRnBhY2thZ2UuanNvbi5qcyclM0IlMjIlMkMlMjJpc0VudHJ5JTIyJTNBdHJ1ZSU3RCUyQyU3QiUyMm5hbWUlMjIlM0ElMjJwYWNrYWdlLmpzb24uanMlMjIlMkMlMjJjb2RlJTIyJTNBJTIyZXhwb3J0JTIwZGVmYXVsdCUyMCU3QiUyMHAlM0ElMjAnanNvbiclMjAlN0QlM0IlMjIlMkMlMjJpc0VudHJ5JTIyJTNBZmFsc2UlN0QlNUQlMkMlMjJvcHRpb25zJTIyJTNBJTdCJTIyZm9ybWF0JTIyJTNBJTIyZXMlMjIlMkMlMjJuYW1lJTIyJTNBJTIybXlCdW5kbGUlMjIlMkMlMjJhbWQlMjIlM0ElN0IlMjJpZCUyMiUzQSUyMiUyMiU3RCUyQyUyMmdsb2JhbHMlMjIlM0ElN0IlN0QlN0QlMkMlMjJleGFtcGxlJTIyJTNBbnVsbCU3RA==.

Feature Proposal

When a module is fully aliased (reexported) through an entry point when that module itself is not an entry point, it should be possible to effectively treat the aliasing entry point as the primary source from a code splitting perspective, resulting in fewer separate chunks in these sorts of alias cases. A heuristic might be to make it that the first alias module in the entry point list in order becomes the primary module source by proxy.