Remove Package.resolved to reveal build errors caused by upstream changes

Checking in `Package.resolved` for library packages is discouraged
because it can mask failures caused by new versions of upstream
dependencies.

When `containerization` is built in CI, or when tests are run inside
a checked-out local copy of the repository, `Package.resolved` pins
all its dependencies to fixed versions.  New versions of upstream
dependencies wil not be tested unless `Package.resolved` is explicitly
updated.

When `containerization` is built as a dependency of a end-user
project, its `Package.resolved` file is ignored.   Instead, the
dependency constraints from `containerization`'s `Package.swift`
file are combined with those of the project and any other library
dependencies, so SwiftPM or Xcode can find a set of mutually compatible
packages.  This can lead to new versions of `containerization`'s upstream
dependencies being used, even though those versions have never been tested
in CI.

The effects of this can currently be seen, because `swift-nio` has
changed upstream, breaking packages which depend on `containerization`
but not breaking `containerization`'s own CI.    `swift-nio` 2.86.1
removes the `NIOFileSystem` product (apple/swift-nio#3370)
which is not yet ready to be public.   CI builds of `containerization` are
not affected by this change because `Package.resolved` pins `swift-nio` to
an older version:

    {
      "identity" : "swift-nio",
      "kind" : "remoteSourceControl",
      "location" : "https://github.com/apple/swift-nio.git",
      "state" : {
        "revision" : "34d486b01cd891297ac615e40d5999536a1e138d",
        "version" : "2.83.0"
      }
    },

In comparison, creating a new package which uses `containerization` shows
that the library can no longer be built:

    % swift package init --type executable
    Creating executable package: test
    Creating Package.swift
    Creating Sources
    Creating Sources/test/test.swift
    % cat > Package.swift <<EOF
    heredoc> // swift-tools-version: 6.2
    // The swift-tools-version declares the minimum version of Swift required to build this package.

    import PackageDescription

    let package = Package(
        name: "test",
        platforms: [
            .macOS(.v26),
        ],
        dependencies: [
            .package(url: "https://github.com/apple/containerization", from: "0.7.2"),

        ],
        targets: [
            // Targets are the basic building blocks of a package, defining a module or a test suite.
            // Targets can depend on other targets in this package and products from dependencies.
            .executableTarget(
                name: "test",
                dependencies: [
                    .product(name: "Containerization", package: "containerization"),
                ]
            ),
        ]
    )
    EOF
    % swift build
    ...
    /private/tmp/test/.build/checkouts/containerization/Sources/ContainerizationOCI/Client/RegistryClient+Fetch.swift:25:8: error: no such module 'NIOFileSystem'
     23 |
     24 | #if os(macOS)
     25 | import NIOFileSystem
        |        `- error: no such module 'NIOFileSystem'
     26 | #endif
     27 |

Removing `Package.resolved` causes the same error to occur in a clean build of `containerization`.

Author: euanh

.gitignore

@@ -11,6 +11,7 @@ DerivedData/
 workdir/
 installer/
 .venv/
+Package.resolved
 test_results/
 *.pid
 *.log