refactor generation of required environment variables in module files

[lexming]

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Goal is to lay groundwork to be able to fix #3331 and add new option to control which search path variables are added in modules by simplifying the code injecting environment variables to module files.

This PR should not alter the resulting module files to what is currently generated by EB. If it changes the list of environment variables it should have no effect (e.g. because those paths are empty).

Simplification of logic handling environment variables for module files:

1. replace obscure rules defining which search paths require populated dirs with:
   • all modules require search paths that point to existing files or non-empty dirs (fixes #3504)
   • fake modules can have search paths to empty dirs
2. replace obscure rules defining which search paths require files in their top level directory with:
   • by default all search paths to directories can be populated at any level (recursive check)
   • this setting is now controlled by top_level_file attribute of ModuleEnvironmentVariable
3. replace convoluted rules to handle symlinked lib64 that generate duplicate paths and need explicit exceptions with:
   • paths to lib64 are always ignored if lib64 is a symlink to lib
   • paths to lib are always ignored if lib is a symlink to lib64
   • remove lib32 from list of library search paths
4. allow simple updates of environment variables in child EasyBlocks, replacing the clunky:

   def make_module_req_guess(self):
       guesses = super(EB_Clang, self).make_module_req_guess()
       guesses.update({'SOME_VAR': ['some/path]})
       return guesses
   

   with an elegant:

   self.module_load_environment.SOME_VAR = ['some/path']
   

Changelog:

• add ModuleEnvironmentVariable pseudo-dataclass to easybuild.tools.modules to hold definitions of environment variables for modules
• add ModuleLoadEnvironment singleton class to easybuild.tools.modules to hold environment definition for modules on load
• deprecate EasyBlock.make_module_req_guess in favor of ModuleLoadEnvironment
• add LibSymlink enum to easybuild.framework.easyblock to define possible linkage states of lib directories
• add install_lib_symlink attribute to EasyBlcok to keep track of symlink status between lib dirs
• add globals to easybuild.tools.config that define known directories for binaries, libraries and headers
• avoid changing the current working directory of the main process in easyblock.make_module_req by working with absolute paths instead