add support for installing hidden module files
implement and use get_hidden_modname
implement 'exists' method for module tools using 'show' subcommand, use 'exists' in 'skip_available'
add support for dependencies installed as hidden modules
enhance unit tests
fix and enhance broken test
add test hidden module
make sure install dir is not hidden, factor duplicate code in ActiveMNS, fix unit tests
escape module name before including it into regex
redefine ModulesTool.exist method to check whether (hidden) modules exist
fix remarks
rename 'hidden' method argument in ActiveMNS to 'force_visible' to make it easier to grok
Merge pull request #1009 from boegel/hidden

add support for installing hidden modules and using them as dependencies (WIP)