TrigOutputHandling+TrigCostMonitorMT: cmake fixes

Some cmake fixes for TrigOutputHandling and TrigCostMonitorMT:

• Remove atlas_depends_on_subdirs
• Add flake8 checking
• Cleanup library dependencies
• Add interface library for public headers

This merge request affects 2 packages:

• Trigger/TrigMonitoring/TrigCostMonitorMT
• Trigger/TrigSteer/TrigOutputHandling

Adding @tamartin ,@rbielski as watchers

added DQ Trigger master review-pending-level-1 labels

CI Result SUCCESS (hash b3ff7c65)

	Athena	AthSimulation	AnalysisBase	AthGeneration
externals
cmake
make
required tests
optional tests

Full details available on this CI monitor view
Athena: number of compilation errors 0, warnings 0
AthSimulation: number of compilation errors 0, warnings 0
AnalysisBase: number of compilation errors 0, warnings 0
AthGeneration: number of compilation errors 0, warnings 0
For experts only: Jenkins output [CI-MERGE-REQUEST-CC7 13314]

The changes look good to me, and CI passes. Approving.

Xiaozhong (L1)

added review-approved label and removed review-pending-level-1 label

merged

mentioned in commit 106c5aa0

added sweep:ignore label