Introduce command like gn refs

gn refs <out_dir> (<label_pattern>|<label>|<file>|@<response_file>)*
        [--all] [--all-toolchains] [--as=...] [--testonly=...] [--type=...]

  Finds reverse dependencies (which targets reference something). The input is
  a list containing:

   - Target label: The result will be which targets depend on it.

   - Config label: The result will be which targets list the given config in
     its "configs" or "public_configs" list.

   - Label pattern: The result will be which targets depend on any target
     matching the given pattern. Patterns will not match configs. These are not
     general regular expressions, see "gn help label_pattern" for details.

   - File name: The result will be which targets list the given file in its
     "inputs", "sources", "public", "data", or "outputs". Any input that does
     not contain wildcards and does not match a target or a config will be
     treated as a file.

   - Response file: If the input starts with an "@", it will be interpreted as
     a path to a file containing a list of labels or file names, one per line.
     This allows us to handle long lists of inputs without worrying about
     command line limits.

Examples (target input)

  gn refs out/Debug //tools/gn:gn
      Find all targets depending on the given exact target name.

  gn refs out/Debug //base:i18n --as=buildfiles | xargs gvim
      Edit all .gn files containing references to //base:i18n

  gn refs out/Debug //base --all
      List all targets depending directly or indirectly on //base:base.

  gn refs out/Debug "//base/*"
      List all targets depending directly on any target in //base or
      its subdirectories.

  gn refs out/Debug "//base:*"
      List all targets depending directly on any target in
      //base/BUILD.gn.

  gn refs out/Debug //base --tree
      Print a reverse dependency tree of //base:base

Examples (file input)

  gn refs out/Debug //base/macros.h
      Print target(s) listing //base/macros.h as a source.

  gn refs out/Debug //base/macros.h --tree
      Display a reverse dependency tree to get to the given file. This
      will show how dependencies will reference that file.

  gn refs out/Debug //base/macros.h //base/at_exit.h --all
      Display all unique targets with some dependency path to a target
      containing either of the given files as a source.

  gn refs out/Debug //base/macros.h --testonly=true --type=executable
          --all --as=output
      Display the executable file names of all test executables
      potentially affected by a change to the given file.