Add import/export capabilities to ShEx

[labra]

When defining large schemas it may be necessary to have some mechanism to import shapes from other schemas.

Currently, ShEx contains the shapeExternal mechanism to reference a shape by an IRI and let the ShEx processor search that shape in some external place.

The definition of shapeExternal is quite minimalistic:

an external shape (ShapeExternal) is an extension mechanism to externally define e.g. functional shapes or prohibitively large value sets.

whose semantic is defined as:

Se is a ShapeExternal and implementation-specific mechansims not defined in this specification indicate success.

Although this minimalistic definition can be good to avoid over-specification, we may consider adding a more general import/export mechanism to ShEx similar where one can:

• Declare from a ShEx schema that he wants to import the shapes declared in another ShEx schema
• Declare in a ShEx schema shapes that can be exported

If we were using RDF syntax for ShEx, one possibility would be to reuse owl:imports declarations although I think it would not be a good idea to mix another vocabulary here.

This concern about importing and modularizing ShEx declarations appears in practice. For example, in this question by @andrawaag in gitter

[ericprud]

It is our belief that the semantics in ShEx 2.1 §5.6 ShEx Import address this. Please close this issue if you agree.

[labra]

I close the issue as the import declaration addresses it