Support for imports

[Leosimetti]

This PR introduces a new interface with support for imports (analyzeSourceCodeDir) for analyzing multiple files.

Import Definition

In EO, imports refer to the following constructs at the top of the file:
+alias bndAlias pkgName.bndSrc
Each import consists of three parts:

1. Optional alias - bndAlias (binding name can be changed when its imported);
2. Optional package name - pkgName (packageless objects are stored are a part of the global package with no name);
3. Path to the imported binding -bndSrc (can be either a top-level object or a path to some nested binding).

Supported Functionality

The following import functionality is currently supported:

• Importing of packageless bindings
  +alias objName
+alias objName.pathToNestedBnd

• Importing of bindings with package
  +alias pkgName.srcPath

• Aliasing in Imports
  +alias name pkgName.srcPath

• Sharing of top-level objects within the same package
  File1:

  +package pkg
  [] > a
  

  File2:

  +package pkg
  [] > b
    a > @
  

Unsupported Features

The following is NOT supported:

• Recursive import fetching (imports of imports)
  Notice how object a is also added to File3
  File1:

  +package pkg1
  [] > a
  

  File2:

  +package pkg2
  +alias pkg1.a 
  [] > b
    a > @
  

  File3:

  +alias pkg2.b
  [] > c
    a > @
  [] > d
    b > @