Add support for a relative offset within ast.SourceInfo

[TristonianJones]

Introduce support for a base line and column to be computed
by querying offset 0 on the common.Source value provided
to NewSourceInfo.

Previously, the common.Source was used directly within the
parser and implementations could override the behavior of
OffsetLocation and LocationOffset to produce values which
treated the parse input as relative source in some larger context;
however, with the shift away from proto (#789) and toward more
uniform and featureful internal data types, the ast.SourceInfo
object attempted to unify the id to offset relationship and removed
this feature on accident.