If a rule isn't part of any component, the build will fail.
The enforcement will be efficient for the `linux_os` benchmark.

Also introduces a `ssg.components` module to the project which
can be used later in other tools and build system to work with
component files data.

This commit adds a simple test case `test_components.py` which
tests that
- components don't map to rules that don't exist
- all rules are mapped to at least 1 component
- the rule is assigned to component according to templates or
  template parameters

This test checks if a rule is mapped to component if it uses
a package platform or other platforms that can be easily used
to determine to which component the rule should belong to.

This commit extracts method `__process_rule` to reduce the code
complexity of the original method, as suggested by CodeClimate.

This aims to address the CodeClimate code complexity problem
in the `main()`.

CodeClimate emits a warning if there is too many return
statements in a single function.

This change reduces code duplication.

Products can decide to use components or they can define their own set
of components. Products specify a path to the directory with component
files by the `components_root` key in the `product.yml`.
This allows products like OCP to use components.

Extract function get_rule_to_components_mapping and move it to
the ssg.components module in order to simplify the BuildLoader
class.

The reason is to avoid code duplication.

This change will add the mapping of a rule to component to the
resolved rules. Developers that build the content will be able
to see all components that the given rule is assigned to in
the resolved rule yamls.

This will prevent everyone from running the script without its
parameters.

This will allow developers to add additional tests for component
mapping when a component assignement will be depending on the
template used in the rule.

If this this behavior cannot be overridden using the world "must"
or "shall" would be better. This applies to all the reasons in this file.

Addressing:
 loader.process_directory_tree(subdir)
  File "/__w/content/content/ssg/build_yaml.py", line 1265, in process_directory_tree
    self._load_group_process_and_recurse(start_dir)
  File "/__w/content/content/ssg/build_yaml.py", line 1259, in _load_group_process_and_recurse
    self._process_rules()
  File "/__w/content/content/ssg/build_yaml.py", line 1383, in _process_rules
    if not self._process_rule(rule):
  File "/__w/content/content/ssg/build_yaml.py", line 1372, in _process_rule
    rule.components = self.rule_to_components[rule.id_]
TypeError: 'NoneType' object has no attribute '__getitem__'

Avoid code duplication in ssg/components.py. The mapping functions
are similar, they differ only in attribute and the type of the
dicitionary values. By unifying the code into a single private function
we can avoid this duplication.