Improve loading of submodules

[DmitryScaletta]

1. Don't load submodules with not installed optional dependencies

Some packages may have optional dependencies.
If there was an error during loading a submodue, and it's a missing optional dependency error, just skip this submodule.

Closes #1937

2. Skip submodules if they have already been loaded as a part of main module

Example: some-module

package.json

{
  "exports": {
    ".": {
      // ...
      "default": "./index.js"
    },
    "./submodule": {
      // ...
      "default": "./submodule.js"
    }
  }
}

index.js

export * from './submodule';

submodule.js

export const submodule = () => {};

The main module exports all from the submodule.
And the submodule exports a function with the same name as a submodule name.
So when loading the submodule, lib already have a property submodule.

Closes #1938

• tests and linter show no problems (npm t)
• tests are added/updated for bug fixes and new features
• code is properly formatted (npm run fmt)
• description of changes is added in CHANGELOG.md
• update .d.ts typings

[DmitryScaletta]

GitHub is lagging. Can't change the base branch.