Refactor structs defined in fleetpkg internal package if possible

In #3213 a new `elastic-package modify` command has been added into `elastic-package` that required to create new internal packages (`fleetpkg` and `yamledit`).

There are some structs in `fleetpkg` that look similar to others already defined in `elastic-package`-

For instance:
- `Manifest` in `fleetpkg` (https://github.com/elastic/elastic-package/pull/3213/changes#diff-7880dbca868888be0062a986f730fcbb094076bd95f397070f6e068e83c142b5R31) and `PackageManifest` in `packages` (https://github.com/elastic/elastic-package/blob/1c096f5f836de67ca842e655f7b7fae391272689/internal/packages/packages.go#L198)
- `Datastream` in `fleetpkg` (https://github.com/elastic/elastic-package/pull/3213/changes#diff-7880dbca868888be0062a986f730fcbb094076bd95f397070f6e068e83c142b5R52) and `DatastreamManifest` in `packages` (https://github.com/elastic/elastic-package/blob/ba9289e88d0a84c5f038b2dea6b0a0aafc49434a/internal/packages/packages.go#L243)

Refactor those structs from `fleetpkg` if possible to use existing structs in `packages` package. To be checked if there could be breaking changes.

Related thread: https://github.com/elastic/elastic-package/pull/3213#discussion_r2731139383

Start by comparing the structs in the internal/fleetpkg package with PackageManifest and DatastreamManifest in internal/packages/packages.go. Review the elastic-package modify command and the related discussion from pull request #3213, then check whether reusing the existing packages types introduces breaking changes. Done means duplicate structs are removed or justified without breaking current behavior.

Written by the indexing model from the issue text.