[Feature] Add package source mapping to package download

[Nigusu-Allehu]

Bug

Fixes: NuGet/Home#14607

Description

This PR

• Adds package source mapping to the package download command
• Here is the design spec https://github.com/NuGet/Home/blob/dev-nyenework-install/accepted/2025/dotnet-package-download.md#source-selection-and-package-source-mapping
• Also update the test mock server to ignore file cases so that it is able to find files despite cases. Before this update the server was not able to find the package files in Linux if the casing is different.

TLDR

• When --source is specified, the command now prioritizes the provided source(s) and skips package source mapping
• When --source is not specified, the command reads sources from nuget.config and applies package source mapping to determine which source to use for each package.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.

[Nigusu-Allehu]

I will mark this PR ready for review, once the following design spec NuGet/Home#14495 gets approved. Or will update the PR if there is design changes.

[Nigusu-Allehu]

Also waiting on #6804 to merge first

[aortiz-msft]

We are missing a test with no --source, mapping -> A,B, B has latest -> retrieves the latest package from B.

[aortiz-msft]

I'm finding this method really hard to read/maintain. There's too many nested control statements and three different code paths for a return statement. One suggestion would be to factor the foreach statement out into its own helper function. Ideally, there should be a single return statement.