Bugfix/10 inconsistent modules status

[gmazzap]

• Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines

• [x] Tests for the changes have been added (for bug fixes/features)

• [x] Docs have been added/updated (for bug fixes/features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Bug fix / improvement.

• What is the current behavior? (You can also link to an open issue here)

See #10

• What is the new behavior (if this is a feature change)?

See #10

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No.

• Other information:

Besides of what described in #10 I've also implemented an additional status: MODULE_SKIPPED.

As I also documented in the README, this status is used in two cases:

• when the Module class only implements Module interface and not a more specific interfaces like ServiceModule or ExecutableModule, etc
• when even implementing a more specific interface, the class return no services/factories/extensions e.g. a ServiceModule whose services() method returns an empty array.

I think this is more important now that we have more reasons for a module to not be marked as added, and it takes into account the issue of classes only implementing Module interfaces, that were an edge-case not handled in any way by the package. At least, now they leave a trace.

[gmazzap]

@Chrico @pablok34 I addressed all the comments. Please have a look again at the PR when you've time.