mark entry points as external

esbuild doesn't allow marking entry points as externals. this is reasonable, but there is another use case.
an entry point could be imported within another entry point, in this case, we need to dynamically import it as if it built separately.

say we have two entry points a.ts and b.ts, and a.ts marked as external
both entry points will be built separately, ignoring the fact that each one may be marked as external in another context.

as b.ts imports a.ts, a.ts could be safely marked as external.

conclusion:
any file could be introduced as an entry point and marked as an external in the same configuration.

The issue names no implementation files or tests. Start by reproducing the two-entry-point case with a.ts and b.ts, where b.ts imports a.ts and a.ts is external; done means the configuration accepts any entry point as external and dynamically imports it when needed.

Written by the indexing model from the issue text.