Change package structure for Kotlin code under `buildSrc`

[alexander-yevsyukov]

The package structure for the dependency objects we use instead of Gradle Version Catalog is mouthful, e.g., io.spine.internal.dependencies.spine.Spine. We still have not decided on migrating to Version Catalog for dependencies and we do have other code. Let's improve the code structure under buildSrc before it's exposed to wider audience.

Suggested repackaging (jupdated after comments and discussions):

• io.spine.internal.dependency -> io.spine.dependency with the sub-packages compile, test, and lib.
• io.spine.internal.dependency.spine -> io.spine.dependency.local.
• io.spine.internal.gradle -> io.spine.gradle.
• io.spine.internal.markup -> io.spine.docs.

[alexander-yevsyukov]

Another aspect to discuss is separation of compile-time dependencies, libraries, and test frameworks and libraries. So, it could be:

io.spine.build.dependency.compile
io.spine.build.dependency.lib
io.spine.build.dependency.test

Some of the dependencies we have are transitive from the build-time tools (e.g. Plexus). They are still libraries by their nature and should go into the lib package.

[alexander-yevsyukov]

Alternative names for io.spine.build.dependency.internal:

• io.spine.dependency.module — modules of the Spine SDK.
• io.spine.dependency.artifact — artifacts of the Spine SDK.
• io.spine.dependency.local — dependencies within the SDK.

I've removed the build infix in the list above deliberately to see how it would look.

[alexander-yevsyukov]

The package io.spine.internal.markup should be renamed to io.spine.docs.

The only class currently in the package is MarkdownDocument. It's a bit surprising to see a Markdown-something in the markup package.

Also, we may have other utilities related to the documentation (in a non-Gradle way) coming to this package.