cmd/{build,check}: respect capabilities for parsing (#5326)

Before, the capabilities were plumbled through in most places:

1. checking which builtins exist
2. passed along to the optimizer
3. passed along to the planner

But they hadn't been passed along to the file loader. As such, it could not
pass the caps along to the parser either. This is now done, but adding a new
method to the FileLoader interface.

Fixes #5323.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>