Added metapackage validation module
Split metapackage validation into small functions
metapackage.py: follow sphinx documentation rules

See: http://sphinx-doc.org/markup/desc.html#info-field-lists
relocate package_exists_at to packages.py
Replaced triple single quotes with double quotes
Use metapackage CMake template from cmake.py
metapackage.py: Added :py:class cross references
parameter for warnings out in validate_metapackage
package.py: use metapackage.py for checks
metapackage.py: Fix circular dependency
test_metapackage.py: capture stderr with stdout
move package_exists_at to package.py from packages.py
moved logic into package.py from metapackage.py to fix circular dependency