New modulecmd detection algorithm.

[skosukhin]

Sometimes modulecmd is not in the PATH and the only way to find it is to parse the corresponding shell function. This is what this PR offers.

[mamelara]

@skosukhin any reason why this was closed? It's pretty useful

[skosukhin]

@mamelara I've closed this because we can't be sure that the executable called by the 'module' shell function is actually what we expect it to be. I've stumbled upon a very strange configuration of the Environment Modules where the 'module' shell function called not 'modulecmd' but some additional wrapping script, which acted differently. So, to make thing simple and straightforward I decided that it was much better just to let users know that they need to add 'modulecmd' executable to the PATH and it was up to them to make sure that it was the 'right' one.