On Fuchsia, we can now get executable VMOs from trusted backing
filesystems.  This allows us to remove the use of replace_as_executable
in favor of opening files with `fdio_open_fd_at` with the
`OPEN_RIGHT_EXECUTABLE` flag and getting VMOs by calling
`fdio_get_vmo_exec`.

By moving the responsibility for executability into the filesystem, we
are able to remove `deprecated-ambient-replace-as-executable` from
component manifests for non-JIT runners (the JIT runners still call
replace_as_executable in Dart's allocator).  It wasn't abundantly clear
whether .cmx files for tests were used purely in AOT runtime
environments or also saw JIT usage, so I left those as-is.

For context: this is a second attempt at flutter#16690, which was reverted
because it broke the Dart JIT runner.  The primary difference is that
this time around, we correctly handle absolute vs relative paths,
depending on whether library loading bottoms out in `fdio_open_fd` or
`fdio_open_fd_at`.  I've added additional assertions to help ensure any
new usages use the correct shape of path.

Testing: I verified locally that the flutter product runner works on
Astro, and also successfully ran the Dart JIT example test (which was
the thing blocking the google3 roll with the previous attempt at this
patchset).