Refactor build tasks to run common task as a dependency

With the introduction of a custom task for cleaning the output on rebuild (`CleanExcelAddIn`) we've introduced a class `BuildTaskCommon` that figures out which files are going to be generated in the output.

Whilst this works fine today, it's not very efficient, as the output of this class is needed for both `CleanExcelAddIn` and `CreateExcelAddIn`, and gets executed twice when, really, we should only be executing this once.

My current thinking is that we should create a base `PrepareExcelAddIn` task that will output the files that are going to be generated in the output, and have the `CleanExcelAddIn` and `CreateExcelAddIn` tasks depend on `PrepareExcelAddIn`'s output.

As part of the work with #18 and #19 I'm going to introduce a task that updates the debug settings for the project being built, and that task also needs the features in `BuildTaskCommon` and therefore would benefit from `PrepareExcelAddIn` as well.