fix linkDuplicates for symlinks

[connectdotz]

Summary
addressing #5421.

src:

1. package-linker: fixed a bug that should have skipped hardlinking the symlinks. While this is not a nohoist specific bug, the fact that linked modules can live in multiple locations surfaced this bug.
2. Config: separated error from warning in extracting workspace config so we can turn off the nohoist via workspaces-nohoist-experimental independent of nohoist config in package.json.

__tests__:

1. _helpers: added ability in runner to disable nohoist in Config. This enable tests easily switch between hoist/nohoist without changing the fixture's package.json.
2. tests and fixtures: added related tests in __tests__/commands/install/integration-deduping.js and corresponding fixtures.

Test plan

1. all existing and new tests should pass
2. the sample project in Workspaces nohoist not working with --link-duplicates #5421 should pass.

[connectdotz]

the test-macos-node6 and test-macos-node8 have been stuck over 20 hours... looks like the other PR have suffered the similar fate for 4 days... can someone from the yarn core team take a look of the task queue...?

[connectdotz]

good, tests finally unstuck. @bestander @rally25rs, I didn't see reviewer assigned for this, just wondering if we can get some traction

[bestander]

Love it! Thank you, @connectdotz.

[dannycochran]

@arcanis is there a planned 1.5.2 release? If so, can we include this change? It would make a world of difference for folks using workspaces.

[arcanis]

It should happen soonish but I can't give you an exact date. In the meantime, the nightlies are the recommended way to get features already merged in the trunk:

https://yarnpkg.com/en/docs/nightly

[dannycochran]

@arcanis -- Thank you for the response. I'm aware of the nightly build option but it's hard to prescribe that solution for a team of engineers.

It would be nice if there could be a release cadence for yarn, maybe every 2 weeks? I believe that's what npm does. I say this just as a friendly suggestion and understanding that you all are probably quite busy :-)