With this change, users are now able to run View AST command in
vscode within vscode workspaces that do not include the core libraries.
The relevant core library only needs to be installed in the package
cache.
Currently, we have a number of assertions in the codebase and certain
assumptions about the AST. These don't always hold, sometimes leading to
a crash in the extractor.
The crashes leave incomplete TRAP files that cannot be imported into the
database.
With this change, we still get those incomplete TRAP files, but we also
get a database in the end (even thoough it is also incomplete as we
cannot import everything).
Although this wasn't (as far as I know) causing any performance issues,
it was making the join-order badness report quite noisy, and so I
figured it was worth fixing.
Before:
```
Tuple counts for RegexTreeView::RegExpBackRef::getGroup#dispred#f0820431#ff/2@d3441d0b after 84ms:
1501195 ~3% {2} r1 = JOIN RegexTreeView::RegExpTerm::getLiteral#dispred#f0820431#ff_10#join_rhs WITH RegexTreeView::RegExpTerm::getLiteral#dispred#f0820431#ff_10#join_rhs ON FIRST 1 OUTPUT Rhs.1 'result', Lhs.1 'result'
149 ~0% {5} r2 = JOIN r1 WITH RegexTreeView::RegExpBackRef#class#31aac2a7#ffff ON FIRST 1 OUTPUT Rhs.1, Rhs.2, Rhs.3, Lhs.1 'result', Lhs.0 'this'
149 ~1% {3} r3 = JOIN r2 WITH regex::RegexString::numbered_backreference#dispred#f0820431#ffff ON FIRST 3 OUTPUT Lhs.3 'result', Rhs.3, Lhs.4 'this'
4 ~0% {2} r4 = JOIN r3 WITH RegexTreeView::RegExpGroup::getNumber#dispred#f0820431#ff ON FIRST 2 OUTPUT Lhs.2 'this', Lhs.0 'result'
1501195 ~3% {2} r5 = JOIN RegexTreeView::RegExpTerm::getLiteral#dispred#f0820431#ff_10#join_rhs WITH RegexTreeView::RegExpTerm::getLiteral#dispred#f0820431#ff_10#join_rhs ON FIRST 1 OUTPUT Lhs.1 'result', Rhs.1 'result'
42526 ~0% {5} r6 = JOIN r5 WITH RegexTreeView::RegExpGroup#31aac2a7#ffff ON FIRST 1 OUTPUT Lhs.1 'this', Lhs.0 'result', Rhs.1, Rhs.2, Rhs.3
22 ~0% {8} r7 = JOIN r6 WITH RegexTreeView::RegExpBackRef#class#31aac2a7#ffff ON FIRST 1 OUTPUT Lhs.2, Lhs.3, Lhs.4, Lhs.1 'result', Lhs.0 'this', Rhs.1, Rhs.2, Rhs.3
0 ~0% {6} r8 = JOIN r7 WITH regex::RegexString::getGroupName#dispred#f0820431#ffff ON FIRST 3 OUTPUT Lhs.5, Lhs.6, Lhs.7, Rhs.3, Lhs.3 'result', Lhs.4 'this'
0 ~0% {2} r9 = JOIN r8 WITH regex::RegexString::named_backreference#dispred#f0820431#ffff ON FIRST 4 OUTPUT Lhs.5 'this', Lhs.4 'result'
4 ~0% {2} r10 = r4 UNION r9
return r10
```
In this case I opted for a classical solution: tying together the
literal and number (or name) part of the backreference in order to
encourage a two-column join.
After:
```
Tuple counts for RegexTreeView::RegExpBackRef::getGroup#dispred#f0820431#ff/2@b0cc4d5n after 0ms:
898 ~1% {3} r1 = JOIN RegexTreeView::RegExpTerm::getLiteral#dispred#f0820431#ff WITH RegexTreeView::RegExpGroup::getNumber#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.1, Rhs.1, Lhs.0 'result'
4 ~0% {2} r2 = JOIN r1 WITH RegexTreeView::RegExpBackRef::hasLiteralAndNumber#f0820431#fff_120#join_rhs ON FIRST 2 OUTPUT Rhs.2 'this', Lhs.2 'result'
1110 ~0% {5} r3 = JOIN RegexTreeView::RegExpGroup#31aac2a7#ffff WITH RegexTreeView::RegExpTerm::getLiteral#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.1, Lhs.2, Lhs.3, Lhs.0 'result', Rhs.1
146 ~0% {3} r4 = JOIN r3 WITH regex::RegexString::getGroupName#dispred#f0820431#ffff ON FIRST 3 OUTPUT Lhs.4, Rhs.3, Lhs.3 'result'
0 ~0% {2} r5 = JOIN r4 WITH RegexTreeView::RegExpBackRef::hasLiteralAndName#f0820431#fff_120#join_rhs ON FIRST 2 OUTPUT Rhs.2 'this', Lhs.2 'result'
4 ~0% {2} r6 = r2 UNION r5
return r6
```
