[Validator] Validate that apps can import the Pod

[Validator] Compile with -Wincomplete-umbrella

[Init] Only create target definitions for native targets

[PodTargetInstaller] Preserve header_mappings_dir when copying header…

Fix handling of localized files in Pods installed as frameworks

Improve repo lint error messaging

- Revert Use `||=` as previous value of group is used in the line
- Pass the converted pathname into group_for_path_in_group
- Fix spacing in Argument Error output
- Fix wrong variable name in Argument Error output

- Remove duplicate argument check from add_file_reference.
  This will make the stack trace a little less concise, as the private
  group_for_path_in_group will show up as the topmost item. However it makes
  the method more readable.
- Make lproj_regex case-insensitive
- Use `||=` instead of wrapping `||` over multiple lines

It is possible that a pod lib creator includes multiple assets with the same
name, but different file extensions.
Instead of just putting localized files by name, they are now also grouped by
the path of the directory their .lproj folder is in.

For example the library Foo could include a localized Foo.storyboard as well as
a localized Foo.png
Now two separate variant groups would be created for Foo.storyboard and Foo.png
as long as the two files are in different .lproj bundles in different folders.

Test that subgroups that reflect the directory structure are not created for
a file reference unless requested.

When adding a file reference for a file in a *.lproj folder, use a
variant group to group it with corresponding files for other languages.

Grouping is done by the names without extensions of the files inside the
*.lproj folders.
For example: Base.lproj/Foo.storyboard and de.lproj/Foo.strings will
be put together in a variant group called Foo.

Update Xcodeproj and support .hxx headers

improving the performance of Pod::Installer::Analyzer#generate_pod_targets