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module: drop support for extensionless files in ESM

Rodrigo Muino Tomonari requested to merge github/fork/GeoffreyBooth/drop-import-of-extensionless into master

This PR removes support for extensionless files in ES module contexts, per discussion in https://github.com/nodejs/node/pull/31388#issuecomment-575803774. The current ESM implementation that shipped in 12.0.0 supported extensionless files only as main entry points to the Node process, e.g. node extensionless-file, and not via import; support for extensionless files in import was added only last month, in https://github.com/nodejs/node/pull/31021. This PR reverts that one, and goes further to drop support for extensionless files as main entry points. The docs are also updated.

The prior support for extensionless main entry files was there as part of supporting bin files. As a practical matter, the “empty string” extension was designated as ES module JavaScript (equivalent to .mjs) within a "type": "module" scope. This is problematic for adding future support for extensionless files of other formats, such as WASM/WASI.

In the interest of preserving design space for such future support, this PR makes all extensionless files unknown (and therefore throwing) to the ESM loader. Another solution besides https://github.com/nodejs/node/pull/31021 or the implementation’s earlier special casing of extensionless main entry points will need to be found for configuring Node’s ES module loader on how to treat extensionless files of varying formats.

Checklist
  • make -j4 test (UNIX), or vcbuild test (Windows) passes
  • tests and/or benchmarks are included
  • documentation is changed or added
  • commit message follows commit guidelines

cc @nodejs/modules-active-members

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