Make conda installation functions private

[allemangD]

One of the nitpick issues mentioned in Slicer/ExtensionsIndex#2006 (review)

Make the conda installation functions installConda into __private methods so it is clear they shall not be called by third-party code.

The only way to install anything to system should be via the CondaSetUp module UI, or by manually installing a distribution and setting its path. Making these methods private will ensure, or at least make obvious, that other modules shall not automatically install conda distributions without user input.