Fix registry package swizzling when package name casing differs

[fortmarek]

Fix registry package swizzling when package name casing differs

Motivation:

When swizzling dependencies when running swift package --replace-scm-with-registry resolve, package references that have different casing don't get swizzled.

You can find an example of this scenario in the Firebase SDK that boils down to:

import PackageDescription


let package = Package(
  name: "Firebase",
  dependencies: [
    .package(
      url: "https://github.com/google/promises.git", // lowercase
      "2.4.0" ..< "3.0.0"
    ),
  ],
  targets: [
    .target(
      name: "FirebaseCrashlytics",
      dependencies: [
        .product(name: "FBLPromises", package: "Promises"), // uppercase
      ]
    )
  ]
)

The .product(name: "FBLPromises", package: "Promises")should be swizzled to use the google.promises identifier, but because of the different casing, it's not.

Modifications:

When creating the map for packages to be swizzled, make the key lowercase. When we access the map, we again use a lowercase variant of the package name, so we don't unexpectedly miss an entry because of a different casing.

Result:

Packages with different casing are swizzled when using the swift package --replace-scm-with-registry resolve command.

[MaxDesiatov]

@swift-ci test

[plemarquand]

Thanks for the patch, appreciate the contribution! I just have one small comment.

[plemarquand]

@swift-ci please test

[plemarquand]

@swift-ci test