Decouple parser creation from input selection (#6533)

Decouple parser creation from input selection (#6533)

This commit decouples the creation of a `Parser` instance from creating
an `Input` and setting the `Input` on the parser. This is a first step
in refactoring the parser infrastructure. A future PR will split the
parser class into three classes: `Parser`, `ParserState`, and
`InputParser`. The `Parser` and `InputParser` classes will be the
public-facing classes. The new `Parser` class will have methods to
create `InputParser`s from files, streams, and strings. `InputParser`s
will have methods to get commands/exprs from a given input. The
`ParserState` class will keep track of the state of the parser and will
be the internal interface for the parsers. The current `Parser` class is
used both publicly and internally, which is messy.