Python: Fix bad join in `ScopeEntryDefinition`

[tausbn]

Before:

Tuple counts for Essa::ScopeEntryDefinition#class#24e22a14#f/1@45e0d8dh after 10.5s:
2133368   ~1%     {2} r1 = Essa::TEssaNodeDefinition#24e22a14#ffff_03#join_rhs AND NOT Essa::ImplicitSubModuleDefinition#class#24e22a14#f(Lhs.1 'this')
534478950 ~0%     {2} r2 = JOIN r1 WITH Definitions::SsaSourceVariable::getScopeEntryDefinition#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.1 'this', Rhs.1
581249    ~4%     {1} r3 = JOIN r2 WITH Essa::EssaNodeDefinition::getDefiningNode#dispred#f0820431#ff ON FIRST 2 OUTPUT Lhs.0 'this'
                return r3

Let's see if pushing the getDefiningNode join further up improves the number of intermediary tuples. (Intuitively it should, since there should only be one defining node for any given EssaNodeDefinition.)

To do this, we unbind the this.getSourceVariable() part, which encourages the compiler to put this join later.

After:

Tuple counts for Essa::ScopeEntryDefinition#class#24e22a14#f/1@30758cv4 after 300ms:
2133569 ~1%     {2} r1 = SCAN Essa::TEssaNodeDefinition#24e22a14#ffff OUTPUT In.0, In.3 'this'
2133368 ~1%     {2} r2 = r1 AND NOT Essa::ImplicitSubModuleDefinition#class#24e22a14#f(Lhs.1 'this')
2133368 ~0%     {2} r3 = JOIN r2 WITH Definitions::SsaSourceVariable#class#486534ab#f ON FIRST 1 OUTPUT Lhs.1 'this', Lhs.0
2133368 ~0%     {3} r4 = JOIN r3 WITH Essa::EssaNodeDefinition::getDefiningNode#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.1, Rhs.1, Lhs.0 'this'
581249  ~4%     {1} r5 = JOIN r4 WITH Definitions::SsaSourceVariable::getScopeEntryDefinition#dispred#f0820431#ff ON FIRST 2 OUTPUT Lhs.2 'this'
                return r5

Much better (and our intuition is confirmed -- joining with getDefiningNode did not increase the number of tuples).