PowerShell ecosystem: using module and NestedModules support

### Is there an existing issue for this?
- [x] I have searched the existing issues

### Feature description
A part of a series of chunk sized changes.

Scope list for the PowerShell ecosystem work:
- Core functionality: `#Requires -Modules` + `RequiredModules` on PowerShell Gallery.
- Microsoft Artifact Registry (MAR) support.
- `using module` + `NestedModules` declaration support.
- Custom/private registry support.

This issue covers: **`using module` + `NestedModules` declaration support**.

In scope for this issue:
- Parse and update `using module` in `*.ps1` / `*.psm1`.
- Parse and update applicable `NestedModules` entries in `*.psd1` when they reference external versioned modules.
- Reuse existing module-specification parsing and constraint handling behavior.

Related scope issues:
- Core functionality: #15501
- MAR support: #15663
- Custom/private registries: #15665

Start by locating the existing module-specification parsing and constraint-handling behavior. Inspect how *.ps1 and *.psm1 files handle using module, then how applicable external versioned NestedModules entries are handled in *.psd1 files. Done means both declaration forms can be parsed and updated while preserving the existing constraints.

Written by the indexing model from the issue text.