Display alternative provider names in provider panel

## Background

In #701, we introduced a provider panel with a list of providers for a module.

## Problem Statement

The current implementation displays the type, namespace, version constraints and installed version of each provider. Users can only know what **alternative names** are providers available under by finding and reading the relevant part of the configuration (i.e. `required_providers` block).

## Expected User Experience

Users can see any alternative name for each provider in the same pane alongside provider requirements.

## Proposal
* Update the LS workspace command `terraform-ls.module.providers` (introduced in https://github.com/hashicorp/terraform-ls/pull/712) to include a list of alternative names
* Update the client-side panel to include those names

## Notes

* When deciding on the UI details, keep in mind that we also want to display provider aliases (#856).

Start by examining the implementation of the terraform-ls.module.providers workspace command and the client-side provider panel introduced in #701. Trace how provider requirements reach the panel, then include alternative names alongside the existing provider details and verify that the panel presents them clearly with the planned provider aliases in #856.

Written by the indexing model from the issue text.