Compare SOUR @VOID@ vs source record

Just like notes could be inline or SNOTE records, a source migrated from "SOUR Some text" in GEDCOM 5.5.1 could be represented in either of two ways, as
```
1 SOUR @VOID@
2 PAGE Some text
```
or
```
1 SOUR @S1@
0 @S1@ SOUR
1 TITL Some text
```
Thus we may want to treat the above as equivalent.