Add option to identify the source library when merging strings.xml files

[AliSoftware]

This improves the existing an_localize_libs action to adds support for the source_id option.

This option aims to helps identify which library each <string> merged into the main strings.xml came from.

Why

• This is prep work towards the plan drafted in paaHJt-3fg-p2
  • The future goal here would be to later update the merge logic to delete all the old entries that came from a previous merge of a library, before merging the new strings again. That way, keys deleted from the lib's strings.xml in a later version would properly be removed from the app's strings.xml, instead of being kept in the apps' strings.xml forever while not being used nor useful anymore
  • I decided to keep that deletion feature for later though, because implementing that feature would likely require a larger change in the current implementation and logic of the an_localize_libs's helper code, and also might require a deeper analysis of potential consequences before going that route (e.g. what if a string is deleted from a lib later… but that key was still used by the app itself too?). Still, adding an attribute to identify the libs the strings were coming from still felt a useful first step anyway
• But also, it will just help us (both the developers and the platform team) understand where each string from the strings.xml is coming from
  • This will be useful to developer to understand, when a string they see in the WordPress/src/main/res/strings.xml, where the source of truth for that string actually comes from (the app, or a lib?) and thus where to change that value if they need to (no point in changing the copy in the app's strings.xml file if the key will be re-overwritten by the next lib merge!)
  • It will also allow them to better reason about when to use content_override if they do want the key from the app's strings.xml to always take over and not be replace by the same key from a lib in the future (useful for example from a key like app_name not to be overwritten by a merge of the about library if it were to define one as well…)

How

The action now accepts a new key in the Hash passed to its libs_strings_path parameter.
That new key is named source_id and allows the caller to provide an (optional) "identifier" string for the library. If provided, that identifier will be added as the value of a custom a8c-src-lib attribute of the <string …> XML nodes added or updated during the merging of the strings of said library.

You can see the result in action in this draft WPAndroid PR.

This PR also takes the occasion to:

• Fix a crash if the caller provided a nil value for the exclusions key in the Hash
• Use UI.message and UI.verbose instead of puts when the helper printed information about the strings being merged or updated.

[mokagio]

Tested this via wordpress-mobile/WordPress-Android#16280. Worked as described. Nothing to comment 👍

[mokagio]

Minor fix, but always appreciated 👨‍🍳 👌