feat: allow per-submodule naming configuration

[RomainMuller]

Introduces a new .jsiirc.json file leveraged by sumodules defined
using the export * as namespace from './namespace'; syntax. The
.jsiirc.json file is a sibling to what ./namespace resolves to (since
this typically resolves to ./namespace/index.ts, the .jsiirc.json file
must be at ./namespace/.jsiirc.json).

The targets key can je set within this .jsiirc.json document and
include specific configuration for various targets:

{
  "targets": {
    "dotnet": {
      // Only the "namespace" key is considered/supported at this point
      "namespace": "Some.DotNet.Namespace"
    },
    "java": {
      // Only the "package" key is considered/supported at this point
      "package": "some.java.package_name"
    },
    "python": {
      // Only the "module" key is considered/supported at this point
      // The configured module name for a submodule _MUST_ be nested
      // under the root module name for the package.
      "module": "some.module_name"
    }
  }
}

Related to #1286

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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[MrArnoldPalmer]

Should the root package name be required in the .jsiirc.json file? Can it still be assumed so instead the custom submodule config for @scope/jsii-calc-lib/lib would be

{
  "targets": {
    "dotnet": {
      "namespace": "CustomSubmoduleName"
    },
    "java": {
      "package": "calculator.custom_submodule_name"
    },
    "python": {
      "module": "custom_submodule_name"
    }
  }
}

What happens if a user uses a root package name in one file that doesn't match the rest of the package? In effect, it makes sense to allow users to nest their submodule's output arbitrarily DEEPER but does it make sense to allow them to be output shallower? And if not why not remove that part of the config?

[MrArnoldPalmer]

Can the 'non-nullability' of these fields not be represented in the assembly spec because of the presence of the dotnet target isn't guaranteed?

[RomainMuller]

Problem is the spec doesn't actually represent per-language configurations... so they're opaque stuff entirely.

[mergify]

Thank you for contributing! ❤️ I will now look into making sure the PR is up-to-date, then proceed to try and merge it!

[mergify]

Merging (with squash)...

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Merging (with squash)...