Add subspec check to 'spec lint' and 'lib lint' commands

This allows to check that each subspec is really independant as it
will build and analyse each subspec.
If a subspec is missing a dependency (even if defined in the spec at
a higher level) this can detect that trying to use only the subspec
will fail.
Add subspecs info to aggregated lint results (similar to platforms)
Add --subspec=NAME option to lib/spec lint

This allows to only check for a given subspec.
Usefull for debugging specs with a lot of subspecs.