LESS version mismatch

Type of Issue

[x] Bug Report
[ ] Feature Request

Description

While converting an Angular 6 library to use ng-packagr, I received this error message:

Unable to interpret argument less-plugin-npm-import - if it is a plugin (less-plugin-less-plugin-npm-import), make sure that it is installed under or at the same level as less

The issue seems to be that ng-packagr requires less@^3.8.0, and my project was already using less@2.3.1. For processing LESS files, the stylesheet-processor.ts uses a direct path into node_modules/less/bin/lessc, which pointed to my project’s local less@2.3.1 package, not the expected less@^3.8.0. Since these two versions have slightly different methods for specifying plugins in CLI arguments, the process failed.

This bug could be avoided by including the expected version of less (currently less@^3.8.0) as a peer dependency.

Related: PR #1060

How To Reproduce

1. Create an angular project using the ng new command
2. Install a local dependency on less@2.3.1
3. Generate a library with ng g library testlib
4. Add a LESS stylesheet to the library
5. Try to build the testlib library with ng build testlib

Expected Behaviour

The ng-packagr package should specify that it expects less@^3.8.0 to be installed as a peer dependency.

Version Information

$ node_modules/.bin/ng-packagr --version
ng-packagr:            4.7.1
@angular/compiler:     6.1.10
rollup:                0.67.4
tsickle:               0.32.1
typescript:            2.9.2

$ npm ls less
+-- @angular-devkit/build-angular@0.6.8
| `-- less@3.9.0
+-- less@2.3.1
`-- ng-packagr@4.7.1
  `-- less@3.9.0

[JoostK]

@alan-agius4 I was just going to type that… 😛

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