pkg_install: make callable from a symbolic macro (#1069)

`pkg_install` wires its internal `_pkg_install_script` rule to a
`py_binary` via the script's *output filename*:

    py_binary(
        name = name,
        srcs = [":" + name + "_install_script"],
        main = name + "_install_script.py",   # <-- file-name-as-label
        ...
    )

`_pkg_install_script` declared that file at analysis time via
`ctx.actions.declare_file(...)`, so the file `<name>_install_script.py`
existed only as an artifact in the rule's `DefaultInfo` — no
loading-time package label was ever registered for it. Legacy macros
tolerated the file-name-as-label shortcut, but in a symbolic macro,
label references must resolve to real loading-time targets in scope.
Bazel errors with:

    no such target '//pkg:<name>_install_script.py': target
    '<name>_install_script.py' not declared in package '<pkg>'
    (did you mean <name>_install_script?)

Promote the script file to a predeclared output via `attr.output()`.
The caller passes `out = <name>_install_script.py`, which registers
`//pkg:<name>_install_script.py` as a real package label at loading
time. `py_binary.main` can then resolve that label in any macro
scope, and the rule impl reads the file through `ctx.outputs.out`
instead of `ctx.actions.declare_file`. No change to the user-facing
`pkg_install` API.