Allow importing VDBs with conflicting model

It should be possible to import a model via "import-vdb" that conflicts with other models also imported via "import-vdb" if the conflicting model has the same version.

This will allow greater model reuse, and not to have duplicated model definitions in multiple vdbs.

Example:

VDB 1:

<code>
<vdb name="OneVDB" version="1">
<description>One VDB</description>

<import-vdb name=ConflictingVDB" version="1"/>
<import-vdb name="OtherVDB" version="1"/>

</code>

VDB 2:
<code>
<vdb name="TwoVDB" version="1">
<description>TwoVDB</description>

<import-vdb name=ConflictingVDB" version="1"/>
<import-vdb name="YetOtherVDB" version="1"/>

</code>

VDB 3:

<code>
<vdb name="ThirdVDB" version="1">
<description>Third VDB</description>

<import-vdb name=OneVDB" version="1"/>
<import-vdb name="TwoVDB" version="1"/>

</code>

Currently we cannot use the ThirdVDB as is, since there is a conflicting VDB (ConflictingVDB) defined in VDBS One and Two.

Since both are using the same version, it should be possible to ignore the conflict (via a property for example).