repl: preload missing constants module

[addaleax]

Checklist

• tests and code linting passes
• the commit message follows commit guidelines

Affected core subsystem(s)

repl, node cli

Description of change

The constants module was previously missing from the list of builtin modules which are available without requiring in the repl and using the --eval command line option. This patch adds it to that list.

[mscdex]

LGTM if CI is ok: https://ci.nodejs.org/job/node-test-pull-request/2449/

[vkurchatkin]

I think it's missing because it's not documented (=private)

[addaleax]

@vkurchatkin Thought so too, until I saw the PR for adding constants.O_NOATIME. It’s actually explicitly mentioned in the docs for fs.open and for the crypto module.

[Fishrock123]

I don't think it's actually private though; if it were, it would be in process.binding() only I think?

[addaleax]

@Fishrock123 I’d consider the _-prefixed files in lib/ private, too, and they can be require()d like any other core module.

My understanding so far was that, essentially, the modules that are considered public are the ones that are documented, and require('constants') is definitely referenced in the docs.

[jasnell]

@ChALkeR ... are you able to pull together some stats on how many modules are using require('constants')?

[ChALkeR]

@jasnell Done: https://gist.github.com/ChALkeR/b4d8cca42890718229f82933c721bbe2.

Most of that is error handling, e.g. ENOTSUP/ENOENT/EPIPE.

[bnoordhuis]

LGTM. require('constants') is in that grey area where it's de facto if not de jure public API. A lot of existing code uses it and you sometimes can't avoid it, e.g., for the SSL_OP_* constants.

[cjihrig]

LGTM

[jasnell]

Yep.. Thanks @ChALkeR! LGTM.
As a next step we should get documentation for this finally also. @nodejs/documentation

[Fishrock123]

I can't think of a reason why we wouldn't document it?

[addaleax]

Putting this on hold because #6534 seeks to deprecate the module completely :)

[addaleax]

Closing in favour of #6534.