* Introduce a new package under ciutil for individual CI systems. Split-out each CI system with env var detection for each.
* Add Bitbucket Piplines env var detection.
* Update changelog with note about adding Bitbucket Pipelines detection.
* Rename the CI system structs.
* Move files from ciutil/systems to ciutil. Un-export some types that don't need visibility beyond the ciutil package.
* Un-export DetectSystem function and the System type.
* Add a test for CI systems which we only know by name and nothing else, i.e. those with just a baseCI implementation.
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