Improve yarn detection

[Keraito]

Issue: Currently, getstorybook uses yarn when it's available on the system even in cases where npm is used in the current project, causing errors. (closes #3384)

What I did

I changed the condition for using yarn from checking yarn --version to looking for a yarn.lock file in the current project.

How to test

• I ran the e2e tests and all of them passed. Testing this with a (additional) fixture seemed excessive based on the fixtures, but please correct me if I'm wrong.

[Keraito]

After pushing I thought of the case where yarn.lock exists, but yarn is not installed on the system. The process will stop as no yarn is available, is that a (rare) case that is storybook's responsibility to cover?

[Hypnosphi]

is that a (rare) case that is storybook's responsibility to cover?

I think it wouldn't hurt to have an additional check

[codecov]

Codecov Report

Merging #3453 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #3453      +/-   ##
==========================================
- Coverage   37.15%   37.14%   -0.02%     
==========================================
  Files         465      465              
  Lines       10316    10320       +4     
  Branches      925      919       -6     
==========================================
  Hits         3833     3833              
- Misses       5933     5963      +30     
+ Partials      550      524      -26

Impacted Files	Coverage Δ
lib/cli/lib/has_yarn.js	0% <0%> (ø)	⬆️
addons/a11y/src/components/Report/RerunButton.js	0% <0%> (ø)	⬆️
...tive/src/preview/components/StoryListView/index.js	0% <0%> (ø)	⬆️
addons/storyshots/src/rn/loader.js	53.33% <0%> (ø)	⬆️
...i/src/modules/ui/components/stories_panel/index.js	20.63% <0%> (ø)	⬆️
lib/ui/src/modules/ui/components/menu_item.js	19.14% <0%> (ø)	⬆️
...tories_panel/stories_tree/tree_decorators_utils.js	45.23% <0%> (ø)	⬆️
addons/info/src/components/types/Shape.js	66.03% <0%> (ø)	⬆️
lib/core/src/server/utils.js	40.47% <0%> (ø)	⬆️
addons/info/src/components/types/OneOfType.js	60% <0%> (ø)	⬆️
... and 67 more

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[danielduan]

I think we should default to using npm unless (yarn is installed AND (has yarn lock OR npm not installed))

We're bound to get issues with these edge cases given the number of users we have.

[Keraito]

I changed the function to behave as specified in #3453 (comment), defaulting to npm unless the condition matches. All e2e tests still pass on my end.

[ndelangen]

This looks mergable, what's wrong with the CI @Hypnosphi ?

[Hypnosphi]

Connection timeout, I've restarted