Can't disable custom rule in nested config file

[jpsim]

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

Describe the bug

When attempting to disable a custom rule defined in a parent configuration from a nested configuration, SwiftLint can't find that rule and prints the message configuration error: 'my_custom_rule' is not a valid rule identifier.

Complete output when running SwiftLint, including the stack trace and command used

$ tree -a
.
├── .swiftlint.yml
├── dir1
│   ├── .swiftlint.yml
│   └── file2.swift
└── file1.swift

1 directory, 4 files
$ cat .swiftlint.yml
custom_rules:
  no_abc:
    name: "Don't use abc"
    regex: 'abc'

$ cat file1.swift
// abc
$ cat dir1/.swiftlint.yml
disabled_rules:
  - no_abc
$ cat dir1/file2.swift
// abc
$ swiftlint
Loading configuration from '.swiftlint.yml'
Linting Swift files at paths ...
configuration error: 'no_abc' is not a valid rule identifier
Valid rule identifiers:
anyobject_protocol
...
yoda_condition
Linting 'file1.swift' (1/2)
Linting 'file2.swift' (2/2)
/path/to/dir/file1.swift:1:4: warning: Don't use abc Violation: Regex matched. (no_abc)
/path/to/dir/dir1/file2.swift:1:4: warning: Don't use abc Violation: Regex matched. (no_abc)
Done linting! Found 2 violations, 0 serious in 2 files.

Environment

• SwiftLint version (run swiftlint version to be sure)? 0.27.0
• Installation method used (Homebrew, CocoaPods, building from source, etc)? Homebrew
• Which Xcode version are you using (check xcode-select -p)? /Applications/Xcode-10.app/Contents/Developer

[jpsim]

Fixed in #2556.