[packchk] Namespaces does not need to be https URLs

[xoriath]

Describe the bug
In the latest packchk 2.9.0, we are now triggering a warning M300

*** WARNING M300: /home/jenkins/agent/workspace/dfp/DFP-PIC32CM-MC/pack-packchk/build/Microchip.PIC32CM-MC_DFP/1.5.204/Microchip.PIC32CM-MC_DFP.pdsc (Line 776) 
  at must use 'https://'

This is caused by the .pdsc fragment

<at:extension xmlns:at="http://www.atmel.com/schemas/pack-device-atmel-extension"
                             schemaVersion="1.0"
                             xsi:schemaLocation="http://www.atmel.com/schemas/pack-device-atmel-extension PACK.DEVICE.ATMEL.EXTENSION.xsd"><!--https://schemas.microchip.com/com/microchip/packs/extensions/at/1.0/PACK.DEVICE.ATMEL.EXTENSION.xsd-->

The warning also triggers on the schemaLocation, which granted is correct, a schema should only load over HTTPS. However the namespace URL itself is meaningless, as it is just a identifier that is a URL by convention. It is also not changeable without breaking the namespace consumers altogether.

To Reproduce

Run packchk -i ARM.CMSIS.pdsc Microchip.PIC32CM-MC_DFP.pdsc

Microchip.PIC32CM-MC_DFP-1.5.204.pdsc.zip

Expected behavior
Warning should be emitted for schemaLocation, but not for namespace URL.

Environment (please complete the following information):

• packchk
• Version 2.9.0
• OS: linux

[xoriath]

Looks like the code itself was introduced in #1109

It's the attribute loop in https://github.com/Open-CMSIS-Pack/devtools/blob/main/tools/packchk/src/CheckFiles.cpp#L257-L261 that issues this specific warning.