McuSupportPlugin: refactor McuPackage

Currently McuPackage is an abstraction that loosely represents a 3rd party dependency. Some dependencies are implicitly set up in the Kit creation functions of McuSupportOptions.

Currently the McuSupportPlugin:

• creates a special McuPackage representing the SDK itself
• pre-generates a list of packages, and reads their values from the QtCreator configuration if exists
• parses json files with kit descriptions, and fills in a McuTargetDescription structure
• matches some loose data from the json files with the pre-generated packages, filling-in missing data with own hard-coded information, and inferring some of the information from other missing data
• Creates McuTarget objects, based on the combination of the packages with the inferred info. When creating these, some new packages are also created on-the-fly.
• The information displayed in the dialog is based on the packages as owned by a given target. Later kits can be created based on that as well.

Should be:

• "Packages" should be rethought as representing one variable each. It could be an environment variable, a cmake variable, a path entry, or something else.
• Each one of those "variable containers" has a separate ID, user-facing description, actual variable name (as needed by cmake), current value, default value.
• They are created "on-demand" as parsed from json, not pre-populated. Since each has a unique ID, they can be stored in a pool and pulled from there when needed if they are used by more than one "target"
• the information that goes inside one has to be pulled from the json kit descriptions, not hard-coded.