Start work on 1:0.2.75
UbuntuDrivers/kerneldetection.py: use the source name only for -lts packages
tests/ubuntu_drivers.py: add more tests for linux-headers checks
UbuntuDrivers/kerneldetection.py: add KernelDetection.get_linux_metapackage()

This also include the relevant tests for the new function
UbuntuDrivers/detect.py: add get_linux()

Also add coverage for both get_linux() and get_linux_headers() in
tests/ubuntu_drivers.py.
releasing 1:0.2.75