1. Handle external functions in other images
  When we find an edge to an external function (already cached in an
  loaded pkgimage), we emit a global variable which we will patch during
  loading with the address of the function to call.

2. Split `.ji` and `.so`
  Validation headers and srctext goes into `.ji`, data and objectcode
  goes into .so.

3. Implement Base.Linking using lld to link on user machines
  On MacOS we are not gurantueed to have a usable `-lSystem` we can
  link against, so we use `-undefined dynamic_lookup`

4. Implement logic for caching and selection

Co-authored-by: Mosè Giordano <giordano@users.noreply.github.com>
Co-authored-by: Tim Holy <tim.holy@gmail.com>

Co-authored-by: Max Horn <max@quendi.de>

Co-authored-by: Michael Schlottke-Lakemper <michael@sloede.com>

Co-authored-by: Michael Schlottke-Lakemper <michael@sloede.com>

Co-authored-by: Alex Ames <alexander.m.ames@gmail.com>

The motiviation here is to allow PkgCacheInspector to both
load the package as well as return the contents of the cache file.
That makes it possible to collect & compare multiple cache files
within the same session.

…ll.a` on Windows (#47864)

The new build provides the import library for `libssp` on Windows, instead of
the static library.

Formerly we only looked for native code in `worklist` modules.
This adds the new "external method extensions" (new methods
owned by the `worklist` but for functions owned by non-`worklist`
modules) and new specialization of external methods.

This reverts commit 6e9a670.