Ignore nonexistent globs

[jszumski]

Prunes podspec patterns that don't match anything on disk. This is needed for Bazel builds with --incompatible_disallow_empty_glob enabled. Invalid patterns typically show up in 2 ways:

• mistakes by the podspec author. Examples from Adyen, AardvarkCrashReport, and Aardvark.

• intentional **/* patterns that are generously expanded into **/*.{ext} for each supported extension. Example from GoogleDataTransport:

  s.source_files = ['GoogleDataTransport/GDTCORLibrary/**/*',
                    'GoogleDataTransport/GDTCCTLibrary/**/*']

  turns into:

  name = "GoogleDataTransport",
  srcs = glob([
      "GoogleDataTransport/GDTCORLibrary/**/*.h",
      "GoogleDataTransport/GDTCORLibrary/**/*.hh",
      "GoogleDataTransport/GDTCORLibrary/**/*.m",
      "GoogleDataTransport/GDTCORLibrary/**/*.mm",
      "GoogleDataTransport/GDTCORLibrary/**/*.swift",
      "GoogleDataTransport/GDTCORLibrary/**/*.c",
      "GoogleDataTransport/GDTCORLibrary/**/*.cc",
      "GoogleDataTransport/GDTCORLibrary/**/*.cpp",
      "GoogleDataTransport/GDTCCTLibrary/**/*.h",
      "GoogleDataTransport/GDTCCTLibrary/**/*.hh",
      "GoogleDataTransport/GDTCCTLibrary/**/*.m",
      "GoogleDataTransport/GDTCCTLibrary/**/*.mm",
      "GoogleDataTransport/GDTCCTLibrary/**/*.swift",
      "GoogleDataTransport/GDTCCTLibrary/**/*.c",
      "GoogleDataTransport/GDTCCTLibrary/**/*.cc",
      "GoogleDataTransport/GDTCCTLibrary/**/*.cpp",
  ]),

  after this change it would be:

  name = "GoogleDataTransport",
  srcs = glob([
      "GoogleDataTransport/GDTCORLibrary/**/*.h",
      "GoogleDataTransport/GDTCORLibrary/**/*.m",
      "GoogleDataTransport/GDTCCTLibrary/**/*.h",
      "GoogleDataTransport/GDTCCTLibrary/**/*.m",
      "GoogleDataTransport/GDTCCTLibrary/**/*.c",
  ]),

The existing test for GlobExpansion already included paths that intentionally didn't exist on disk so I only needed to update its "after" fixture.

Relies on https://github.com/bazel-ios/cocoapods-bazel/pull/79.