* 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.
* Add a var for PRNumber. Add an environment metadata key for PR number.
* Move the detection of PULUMI_CI_SYSTEM into vars.DetectVars(). Set the PRNumber CI property based on respective env vars from each CI system.
* Add Azure Pipelines build variables.
* Add tests for DetectVars.
* Added changelog entry for Azure Pipelines.
* Capture the value of env var being modified for the ciutil unit test, and restore their values at the end of them.
* Simplify the DetectVars function by moving the Pulumi CI system code into the switch-case expression.
* Rename the Pulumi CI system to Generic CI. Include the GenericCI system in the test case for DetectVars.
