Generated Manifest of Build Artifacts

[skmcgrail]

Creates a generated manifest of the build output artifacts. Introduces the ability to handle selection of the minimum required standard library, and minimum version required for a given set of dependencies. This manifest will be used to ultimately remove the responsibilities of generating the go.mod from the smithy-go code base and move that concern into the SDK.

Example generated.json from the smithy-go-codegen-test test code:

{
    "dependencies": {
        "github.com/aws/smithy-go": "v1.3.0",
        "github.com/jmespath/go-jmespath": "v0.4.0"
    },
    "files": [
        "api_client.go",
        "api_op_BadName.go",
        "api_op_GetCity.go",
        "api_op_GetCityAnnouncements.go",
        "api_op_GetCityImage.go",
        "api_op_GetCurrentTime.go",
        "api_op_GetForecast.go",
        "api_op_ListCities.go",
        "doc.go",
        "generated.json",
        "go.mod",
        "types/enums.go",
        "types/errors.go",
        "types/types.go",
        "types/types_exported_test.go",
        "validators.go"
    ],
    "go": "1.15",
    "module": "weather"
}