Allow private fields in preconditions even when the field isn't accessible at call sites.

This is a follow-up to #1582.

The following example is similar to the original bug, where a private field referenced by @RequiresNonNull is not visible across compilation boundaries.

Repro:

package foo;

import org.checkerframework.checker.initialization.qual.UnknownInitialization;
import org.checkerframework.checker.nullness.qual.Nullable;
import org.checkerframework.checker.nullness.qual.RequiresNonNull;

import java.util.function.Supplier;

public class Foo {
  @Nullable private Supplier<String> s;

  @RequiresNonNull("s")
  public String f(@UnknownInitialization Foo this) {
    return s.get();
  }
}

package mainrepropkg;

import foo.Foo;
import java.util.Optional;

public class FlowExprParseError {

  private String t(Foo foo, String type) {
    return Optional.ofNullable(foo.f()).orElse("");
  }
}

1. Create an interface jar for Foo using this tool distributed with Bazel, or just download this copy of the jar I used: https://github.com/typetools/checker-framework/files/1934273/lib-interface.zip

$ ./checker-framework-2.5.0/checker/bin/javac foo/Foo.java 
$ jar cvf lib.jar foo/Foo.class 
$ ijar lib.jar
$ javap -p -cp lib.jar foo.Foo
Compiled from "Foo.java"
public class foo.Foo {
  private java.util.function.Supplier<java.lang.String> s;
  public foo.Foo();
  public java.lang.String f();
}
$ javap -p -cp lib-interface.jar foo.Foo
public class foo.Foo {
  public foo.Foo();
  public java.lang.String f();
}

2. Run the nullness checker on FlowExprParseError.java with foo.Foo on the classpath, observe a flowexpr.parse.error. Based on #1582 I expected a flowexpr.parse.postcondition.error.

$ ./checker-framework-2.5.0/checker/bin/javac -sourcepath : -cp lib-interface.jar -processor Nullness FlowExprParseError.java
FlowExprParseError.java:9: error: [flowexpr.parse.error] cannot parse the expression 's' because identifier not found
    return Optional.ofNullable(foo.f()).orElse("");
                                    ^
1 error

Reproduce the issue with the provided Foo.java, FlowExprParseError.java, interface JAR, and Nullness javac command. Start by tracing handling of @RequiresNonNull expressions when the referenced private field is absent from the interface JAR. Done means the cross-boundary call reports the expected flowexpr.parse.postcondition.error instead of flowexpr.parse.error.