Clarify and limit valid src/res paths and names

Clarify and limit valid src/res paths and names

Backwards compatibility change ahead:

Disallow result and source pathnames that contain a dot (.) as this is
an endless source of pain. Dots may now only be used to group results
and sources. This makes reliable translation from file paths to result
groups and back possible.

Rename the functions doing the validation, add documentation, and make
them available outside of the e2tool module. Improve error messages.

Signed-off-by: Tobias Ulmer <tu@emlix.com>