`INFOPLIST_FILE` will set to wrong file if no Info.plist exist in `SRCROOT`

[blastmann]

According to the project spec, INFOPLIST_FILE will set to the first one found in the SRCROOT if the file does not exist:

INFOPLIST_FILE: If it doesn't exist your sources will be searched for Info.plist files and the first one found will be used for this setting

However, if I generate a Info.plist file in script and set INFOPLIST_FILE in a xcconfig file, this behavior will overwrite my setting and set this value to another file in the folder. Like:

// Debug.xcconfig content
INFOPLIST_FILE = Configurations/ExportSettings/Info.plist

# project.yml content
app:
  platform: iOS
  type: application
  configFiles:
    Debug: Configurations/Debug.xcconfig

// there is another plist file in `SRCROOT/resource.bundle`
// then the project will set `INFOPLIST_FILE` to `SRCROOT/resource.bundle/Info.plist`

I suggest this setting should comply to xcconfig first if it is not set. The searching behavior is weird.