Fix singular names on plurals in Web API service URLs to comply with RESTful practices for all MSI modules

[maghamed]

Use plural nouns
Do not mix up singular and plural nouns. Keep it simple and use only plural nouns for all resources in Magento WEB API.

Sources instead of Source
Stocks instead of Stock
Source-items instead of Source-item
etc

Currently, we have

    <route url="/V1/inventory/source" method="POST">
        <service class="Magento\InventoryApi\Api\SourceRepositoryInterface" method="save"/>
        <resources>
            <resource ref="Magento_InventoryApi::source_edit"/>
        </resources>
    </route>
    <route url="/V1/inventory/source/:sourceCode" method="PUT">
        <service class="Magento\InventoryApi\Api\SourceRepositoryInterface" method="save"/>
        <resources>
            <resource ref="Magento_InventoryApi::source_edit"/>
        </resources>
    </route>
    <!-- Stock -->
    <route url="/V1/inventory/stock" method="GET">
        <service class="Magento\InventoryApi\Api\StockRepositoryInterface" method="getList"/>
        <resources>
            <resource ref="Magento_InventoryApi::stock"/>
        </resources>