feat(version): support combiner source for version check commands

This allows more flexibility and chaining by being able to define multiple sources per pkgbase and chain them together to achieve the final result. This may also be helpful to combine multiple from/to patterns into a chain to achieve the final pkgver to compare against.

To utilize the combiner source, the pkgbase section must be declared as the combiner source. Additionally, individual sections should be added using a quoted table key consisting of the pkgbase followed by the stage name, separated by double colons. For example: ["sudo:stage1"].

Fixes #216 (closed)

Component: pkgctl version check