Add package tags as a semantic entity

[iglosiggio]

This way we can quickly reference and navigate to a given package tag.

Made during the pharo sprint with @matijakljajic

[Ducasse]

Thanks I will have a look.
Would be good to have tests and getXXX is not that idiomatic of Pharo.
I will have a look.

[Ducasse]

I know that the existing code is using this getX stuff.

[iglosiggio]

Yes, we continued the code following what was already there. Now it's probably time to improve it.

OTOH there's no test on this PR and we are adding a new feature. That's not right :P

My rewrite would be:

• Remove the object mutation from the getX methods
• Add a field containing the list of supported "entity providers" (an entity provider is something that respondsTo #value: and expects the token list as argument)
• Make the initial value of this list something like: { [ :tokens | self maybeGetClass ]. [ :tokens | self maybeGetPackage ]. [ :tokens | self maybeGetTag ] } (if we put a bit more work we can migrate the whole computeEntity method to a series of calls like that)
• Then the implementation of computeEntity is just entity := providers detect: [ :provider | provider value: tokens ].