Fix Skypack SDK dependencies for Yarn 2 PnP

[msokk]

Changes

As Yarn 2 is strict with unlisted dependencies, it errors out with following for 3 packages: etag, mkdirp and find-up:

Error: skypack tried to access mkdirp, but it isn't declared in its dependencies; this makes the require call ambiguous and unsound.

Declared the missing utils.ts dependencies in skypack module package.json, used the same version as elsewhere.

Testing

Tested locally, it is hard to do it properly as Snowpack is running Yarn v1

Docs

Bug fix only

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[msokk]

Currently the workaround is to manually define these dependencies in .yarnrrc.yml:

packageExtensions:
  skypack@*:
    dependencies:
      etag: '*'
      find-up: '*'
      mkdirp: '*'

[FredKSchott]

you're right, that's no good. Thank you!