get_filename_component: Document path components more clearly (#14091)

Organize component names in a table to explain each in more detail.
Clearly state that PATH is the directory name.
get_filename_component: Add explicit unit tests

Add test RunCMake.get_filename_component to cover cases of the command.
Remove redundant coverage of these cases from the "complex" tests.
get_filename_component: Add DIRECTORY option (#14091)

The name "PATH" is not precise so add equivalent option "DIRECTORY".
Document the latter as preferred and the former as a legacy alias.
Merge topic 'improve-get_filename_component-names' into next

7d1590e get_filename_component: Add DIRECTORY option (#14091)
ec1ba9b get_filename_component: Add explicit unit tests
df71f96 get_filename_component: Document path components more clearly (#14091)