add module-search-path-headers configuration option to control how modules set search paths to header files

[lexming]

Depends on:

• refactor generation of required environment variables in module files + deprecate make_module_req_guess method in EasyBlock class #4653

This PR adds a new build option and easyconfig parameter called module-search-path-headers to control what environment variables will be added to module files with search paths to headers:

• none: (new setting) no variables added to module files with paths to headers
• CPATH: (default and current setting) modules set CPATH on load
• INCLUDE_PATHS: (new setting) modules set variables C_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJC_INCLUDE_PATH on load with paths to headers

New test added to easyblock suite for this new option.

update 1: easybuild.tools.modules.ModuleLoadEnvironment is enhanced with the capability to hold private attributes that are invisible for the generation of the module environment.

update 2: easybuild.tools.modules.ModuleLoadEnvironment is enhanced with the capability to define arbitrary aliases to ModuleEnvironmentVariable

[boegel]

@lexming please sync this with 5.0.x now that #4653 is merged

[lexming]

This is poorly designed, working on a better approach...

[boegel]

This is poorly designed, working on a better approach...

Poorly designed in what sense?

[lexming]

@boegel in the previous iteration I added a bunch of logic related to the option module_search_path_headers into ModuleLoadEnvironment. This is not ideal, because it does not serve any other purpose than this specific option.
So, I replaced that with a more generic aliases feature that allows to define aliases of environment variables for any purpose. Therefore, handling the specific of module_search_path_headers can be contained in EasyBlock, where it belongs as EasyBlock knows about all build options and easyconfig parameters.

[boegel]

lgtm