linkage_checker: report linkage with system frameworks
linkage_checker: report linkage with system frameworks

Currently, `brew linkage` reports linkage with system frameworks only if
they can be found on the file system. This results in this linkage not
being reported on Big Sur and newer, where system libraries are stored
in the dyld cache instead.

Let's fix that by avoiding silently ignoring system frameworks by moving
them out of `#harmless_broken_link?`. We retain the behaviour desired
from 7228e60da51392b20d55e0c087b2082b86fb3bbf by deferring checking if a
broken library is actually a system framework to just before we add it
to `@broken_dylibs`.

To see how this changes the behaviour of `brew linkage`, here's an
example with this change:

    ❯ brew linkage neovim
    System libraries:
      /System/Library/Frameworks/CoreServices.framework/Versions/A/CoreServices
      /usr/lib/libSystem.B.dylib
      /usr/lib/libiconv.2.dylib
      /usr/lib/libutil.dylib
    Homebrew libraries:
      /usr/local/opt/gettext/lib/libintl.8.dylib (gettext)
      /usr/local/opt/libtermkey/lib/libtermkey.1.dylib (libtermkey)
      /usr/local/opt/libuv/lib/libuv.1.dylib (libuv)
      /usr/local/opt/luajit/lib/libluajit-5.1.2.dylib (luajit)
      /usr/local/opt/luv/lib/libluv.1.dylib (luv)
      /usr/local/opt/msgpack/lib/libmsgpackc.2.dylib (msgpack)
      /usr/local/opt/tree-sitter/lib/libtree-sitter.0.dylib (tree-sitter)
      /usr/local/opt/unibilium/lib/libunibilium.4.dylib (unibilium)

and without this change:

    ❯ brew linkage neovim
    System libraries:
      /usr/lib/libSystem.B.dylib
      /usr/lib/libiconv.2.dylib
      /usr/lib/libutil.dylib
    Homebrew libraries:
      /usr/local/opt/gettext/lib/libintl.8.dylib (gettext)
      /usr/local/opt/libtermkey/lib/libtermkey.1.dylib (libtermkey)
      /usr/local/opt/libuv/lib/libuv.1.dylib (libuv)
      /usr/local/opt/luajit/lib/libluajit-5.1.2.dylib (luajit)
      /usr/local/opt/luv/lib/libluv.1.dylib (luv)
      /usr/local/opt/msgpack/lib/libmsgpackc.2.dylib (msgpack)
      /usr/local/opt/tree-sitter/lib/libtree-sitter.0.dylib (tree-sitter)
      /usr/local/opt/unibilium/lib/libunibilium.4.dylib (unibilium)

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