Don't check generator on PATH if manually specified

[jophippe]

This change addresses item #4005

This changes how the CMake generator gets selected

The following changes are proposed:

• Add a flag to the get best generator logic to tell whether we're selecting a default generator or a manually specified generator.
• If we're selecting a default generator, keep the old behavior and check whether the generator exists on PATH
• If we're selecting a manually specified generator (whether through the cmake.generator setting, the cmake.preferredGenerators setting, or from the kit), we can assume the generator exists and shouldn't check the PATH.

[gcampbell-msft]

If we're going back to, if the user has cmake.generator set, we use it without conditions, I don't think we should have these conditions here, we should simply return true.