pkg_install: make callable from a symbolic macro

[matx-sjacob]

Summary

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 <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?)

Fix

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.

Test plan

• Verified the patch builds locally against a 1.2.0 pin where pkg_install is invoked from inside a symbolic macro (previously failed with the error above).
• Confirm existing pkg_install-related tests still pass in CI.

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