fix: various dependency issues

[merceyz]

Summary

Fixes various dependency issues in order to eventually add Yarn PnP support, these changes aren't PnP specific, they only address dependency issues.

• Add undeclared dependencies
• Use require.resolve for webpack loaders and vuepress plugins, read here for why https://github.com/yarnpkg/berry/blob/ead77bd4612267035f1741a76b18e3b00e9069c7/packages/yarnpkg-doctor/README.md#no-unqualified-webpack-config

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome 83.0.4103.116 (Official Build) (64-bit)
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

A part of this PR is also in #2304 which fixes #2091

x-ref vuepress/vuepress-community#52