[eslint-plugin] Add new import-related rules to the ESLint plugin

[D4N14L]

Summary

• Adds 4 new rules to @rushstack/eslint-plugin
• Update @rushstack/heft-lint-plugin to disable --fix mode when running in --production mode

Details

Adds the following new rules to the @rushstack/eslint-plugin package and consume them in Rushstack:

• @rushstack/no-backslash-imports
  • Rationale: Backslashes are platform-specific and will cause breaks on non-Windows platforms.
  • Default mode in Rushstack: error
• @rushstack/no-external-local-imports
  • Rationale: Avoid consuming dependencies which would not otherwise be present when the package is published.
  • Default mode in Rushstack: error
• @rushstack/no-transitive-dependency-imports
  • Rationale: Consumption of transitive dependencies can be problematic when the dependency is updated or removed from the parent package. Enforcing consumption of only direct dependencies ensures that the package is exactly what we expect it to be.
  • Default mode in Rushstack: warn
• @rushstack/normalized-imports
  • Rationale: Using the simplest possible import syntax is preferred and makes it easier to understand where the dependency is coming from.
  • Default mode in Rushstack: warn

This PR also sets fix mode to false regardless of any provided settings when running in --production mode. This is logged to the user as standard text to avoid producing errors/warnings when the value is set in the Heft configuration for the plugin.

How it was tested

• Added unit tests
• Tested in Rushstack
• Tested in large internal monorepo

Impacted documentation

May require updates to external lint config documentation