package-utils cannot resolve external commands

[ylemkimon]

Describe the bug
packageExists in @webpack-cli/package-utils is used to resolve external commands:

webpack-cli/packages/package-utils/src/packageUtils.ts

Lines 64 to 71 in a515b76

	export function packageExists(packageName: string): boolean {
	try {
	require(packageName);
	return true;
	} catch (err) {
	return false;
	}
	}

However, since external command packages, such as @webpack-cli/info, aren't declared as a dependency, @webpack-cli/package-utils shouldn't be able to require them.

This is especially an issue for Yarn 2 PnP as it requires all dependencies to be explicitly declared to link them (Yarn rulebook).

If Yarn cannot resolve the dependency, it falls back to the top level if pnpFallbackMode is depenencies-only (default) or all. So it's possible to resolve external commands, but not default commands included in the webpack-cli.

For instance, if the dependency tree of a project is:

project
├─D> @webpack-cli/xxx
└─D> webpack-cli
     ├─D> @webpack-cli/package-utils
     └─D> @webpack-cli/info

webpack xxx works but webpack info doesn't work because @webpack-cli/package-utils cannot resolve @webpack-cli/info. webpack xxx doesn't work if pnpFallbackMode: none.

To summarize:

1. @webpack-cli/package-utils cannot resolve default commands (fix: declare default commands as optional peer dependencies #1816)
2. @webpack-cli/package-utils cannot resolve external commands with pnpFallbackMode: none

What is the current behavior?

$ webpack info
Error: @webpack-cli/package-utils tried to access @webpack-cli/info, but it isn't declared in its dependencies; this makes the require call ambiguous and unsound.

Required package: @webpack-cli/info (via "@webpack-cli/info")
Required by: @webpack-cli/package-utils@npm:1.0.1-alpha.4 (via @webpack-cli-package-utils/node_modules/@webpack-cli/package-utils/lib/)

Require stack:
- @webpack-cli-package-utils/node_modules/@webpack-cli/package-utils/lib/packageUtils.js
- @webpack-cli-package-utils/node_modules/@webpack-cli/package-utils/lib/index.js
- webpack-cli/lib/commands/ExternalCommand.js
- webpack-cli/lib/utils/arg-parser.js
- webpack-cli/lib/webpack-cli.js
- webpack-cli/lib/bootstrap.js
- webpack-cli/bin/cli.js
- webpack/bin/webpack.js
    at internalTools_makeError (.pnp.js:21227:34)
    at resolveToUnqualified (.pnp.js:22185:23)
    at resolveRequest (.pnp.js:22277:29)
    at Object.resolveRequest (.pnp.js:22343:26)
    at Function.external_module_.Module._resolveFilename (.pnp.js:21460:34)
    at Function.external_module_.Module._load (.pnp.js:21325:48)
    at Module.require (internal/modules/cjs/loader.js:1025:19)
    at require (v8-compile-cache/v8-compile-cache.js:161:20)
    at packageExists (@webpack-cli/package-utils/lib/packageUtils.js:58:9)
    at Function.run (webpack-cli/lib/commands/ExternalCommand.js:27:22)

[webpack-cli] The command moved into a separate package: @webpack-cli/info

? Would you like to install @webpack-cli/info? (That will run yarn add -D @webpack-cli/info)

To Reproduce

yarn set version berry
yarn init
echo -e 'packageExtensions:\n  "@webpack-cli/package-utils@*":\n    dependencies:\n      "cross-spawn": "^7.0.0"' >> .yarnrc.yml # Workaround for issue already fixed in master
yarn add -D webpack@^5.0.0-rc.0 webpack-cli@^4.0.0-beta.9
yarn webpack info

Expected behavior
webpack info and webpack xxx (xxx is an external command) to run

[ylemkimon]

Fixed via #1822.