Package linting

[openvmp]

Add "cli_lint.py" to implement "pc lint [-r]" to lint the current package or all of them recursively in case of "-r".
Not to be confused with assembly or part linting (to be implemented later).

"cli_lint()" would initialize the context and apply linting rules to all packages (call linting rules one by one passing packages as parameters).
Initialize the list of all linting rules in "lint_package_all.py".
Define the LintPackageRule class to be used by all package linting rules as the best class in "lint_package.py".
Implement a couple of basic linting rules in "link_package_<rule_name>.py".
For example:

• warn about missing "desc" fields in packages, parts and assemblies.
• warn about malformed or unreachable urls if "url" field is present (for now, better not attempt to figure out if "url" is expected to be present or not)