Make file searching dynamically configurable.

This commit extends the previous commit by also making source file
searching dynamically configurable as opposed to forcing the choice when
the data source is created. The `file_exact()` constructor is deprecated
in favor of `file().search(false)`.

src/error.rs

@@ -2,7 +2,6 @@
 
 use std::fmt::{self, Display};
 use std::borrow::Cow;
-use std::path::PathBuf;
 
 use serde::{ser, de};
 
@@ -129,9 +128,6 @@ pub enum Kind {
     /// [`serde::de::Error::duplicate_field()`].
     DuplicateField(&'static str),
 
-    /// A file marked as required was not present.
-    MissingFile(PathBuf),
-
     /// The `isize` was not in range of any known sized signed integer.
     ISizeOutOfRange(isize),
     /// The `usize` was not in range of any known sized unsigned integer.
@@ -464,9 +460,6 @@ impl Display for Kind {
             Kind::DuplicateField(v) => {
                 write!(f, "duplicate field `{}`", v)
             }
-            Kind::MissingFile(v) => {
-                write!(f, "missing file `{}`", v.to_string_lossy())
-            }
             Kind::ISizeOutOfRange(v) => {
                 write!(f, "signed integer `{}` is out of range", v)
             }