Currently, the `ValidateConfig` and the
`createImageCustomizerParameters` functions share some
responsibilities. The `createImageCustomizerParameters` function is
responsible for merging the CLI values with the config values and for
setting up dynamic values (e.g. image UUID). It also validates some
CLI args (e.g. rpm sources). The `ValidateConfig` is responsible for
validating the config. But it also sometimes validates CLI args (e.g.
input image path).  It also needs to not validate unused values (e.g.
config's input image path if the CLI option is specified). This mixed
responsibilities has created a somewhat tight coupling between the two
functions.

This change merges the `createImageCustomizerParameters` function into
`ValidateConfig` so that `ValidateConfig` is responsible for all
validation logic and all config/options merging logic. This will
hopefully make the code easier to maintain by ensuring all related
logic is in one location.

Changes include:

1. Merge `createImageCustomizerParameters` into `ValidateConfig`.

2. Create a new class called `ResolvedConfig` to store all merged
   config/options values.

3. Keep `imageCustomizerParameters` for passing around image metadata.

4. Update `ImageCustomizerOptions`:

   1. Add an `IsValid` function to centralize validity checking of its
      fields.

   2. Change type of `OutputImageFormat` and `PackageSnapshotTime` to
      use API lib's types.

5. Remove `ImageCreatorParameters` since `ResolvedConfig` can be used
   instead.

6. Handle merging package snapshot CLI vs. config in `ValidateConfig`
   instead of in `addRemoveAndUpdatePackages`.

Split the `ImageCustomizerParameters` into two, with a new
`ResolvedConfig` struct taking all the merged config values and any
generated values, and leaving `ImageCustomizerParameters` to handle
passing around image metadata. As part of this, some redundant values in
`ResolvedConfig` are being changed from fields to functions.

This change is being done in preparation for the change that will merge
the config merging logic into the `ValidateConfig` function.

Also, handle the merging of the packages snapshot time when generating
the `ResolvedConfig` instead of deep within the code stack, to match
how other CLI parameters are handled.

Also, refactor some functions to take `ResolvedConfig` instead of a long
list of parameters, to help simplify code. Also, this will make the
upcoming Hierarchical Config changes a little eaiser.

…mage-tools into user/elaine/poc1