The PXT build process (kicked off by pxt serve) has a number of key steps and some special cases. It requires the file pxtarget.json.

Per package,

• enums.d.ts
• shims.d.ts
• dal.d.ts (core package only)

• target id == "core": this is reserved for microsoft/pxt (see http://github.com/microsoft/pxt)
• target id == "common": this is used for targets that serve to bundle a set of directories as common/shared packages (see http://github.com/microsoft/pxt-common-packages).