Add make-location lint for deprecated $(location) make variables (#1484)

## Summary

- Add a new buildifier lint category, `make-location`, that flags
deprecated `$(location)` and `$(locations)` make variables in BUILD
files.
- Dogfood the rule in this repo by replacing existing uses with
`$(execpath ...)` and enforcing the lint via `//:make_location_lint`.

## Background / deprecation context

Bazel's [`$(location)`](https://bazel.build/reference/be/make-variables)
and [`$(locations)`](https://bazel.build/reference/be/make-variables)
make variables are legacy pre-Starlark synonyms for
[`$(execpath)`](https://bazel.build/reference/be/make-variables) and
[`$(rootpath)`](https://bazel.build/reference/be/make-variables). Which
path they expand to depends on the attribute being expanded, which makes
behavior hard to predict and easy to get wrong.

From the [Make
Variables](https://bazel.build/reference/be/make-variables) reference:

> **`location`**: A synonym for either `execpath` or `rootpath`,
depending on the attribute being expanded. This is legacy pre-Starlark
behavior and **not recommended** unless you really know what it does for
a particular rule. See
[#2475](https://github.com/bazelbuild/bazel/issues/2475) for details.

The underlying inconsistency is discussed in
[bazelbuild/bazel#2475](https://github.com/bazelbuild/bazel/issues/2475)
(e.g. `$(location)` expanding to an exec path in some attributes but a
runfiles path in others). Bazel's docs now steer users toward explicit
variables:

- [`$(execpath ...)`](https://bazel.build/reference/be/make-variables) —
path under the execroot where build actions run
- [`$(rootpath ...)`](https://bazel.build/reference/be/make-variables) —
runfiles-relative path for runtime lookup (prefer [`$(rlocationpath
...)`](https://bazel.build/reference/be/make-variables) for
cross-platform runfiles)

This lint nudges BUILD authors toward those explicit forms instead of
the ambiguous legacy alias.

## Changes

- New `make-location` warning in `warn/warn_bazel.go` (BUILD files only;
does not flag `load("location", ...)`).
- Tests, `WARNINGS.md` / `warnings.textproto` docs, and warning-list
updates.
- Repo fixes: `buildifier/BUILD.bazel`, `buildozer/BUILD.bazel`,
`build/build_defs.bzl`.
- New `buildifier_test` target `//:make_location_lint` added to
`//:tests`.

## Test plan

- [x] `bazel test //warn:warn_test
--test_filter=TestMakeLocationVariable`
- [x] `bazel test //warn/docs:docs_test`
- [x] `bazel test //buildifier/config:config_test`
- [x] `bazel test //:make_location_lint`