Fix running terser from external repository. (#125)

I've got a repo which terses some typescript. It works when built inside
the repo, but when the exact same target is built outside the repo, it
fails to build.

load("@aspect_rules_terser//terser:defs.bzl", terser_minified =
"terser")

terser_minified(
    name = name + "__min",
    srcs = [":" + name],
    node_modules = "//:node_modules",
    sourcemap = False,
)

(cd
/dev/shm/bazel-sandbox.a3f391e631c5ea6c72bb0d2c0b90317c117ec497a3759d6de83cbe1ad7ada353/linux-sandbox/6993/execroot/spacecookies
&& \
  exec env - \
    BAZEL_BINDIR=bazel-out/k8-opt/bin \
    COMPILATION_MODE=opt \
    TMPDIR=/tmp \

/home/austin/.cache/bazel/_bazel_austin/install/5309d864f9edb3a2e8380ffc84e6b95c/linux-sandbox
-t 15 -w /dev/shm -w
/dev/shm/bazel-sandbox.a3f391e631c5ea6c72bb0d2c0b90317c117ec497a3759d6de83cbe1ad7ada353/linux-sandbox/6993/execroot/spacecookies
-w /tmp -e /tmp -S
/dev/shm/bazel-sandbox.a3f391e631c5ea6c72bb0d2c0b90317c117ec497a3759d6de83cbe1ad7ada353/linux-sandbox/6993/stats.out
-H -N -U -D
/dev/shm/bazel-sandbox.a3f391e631c5ea6c72bb0d2c0b90317c117ec497a3759d6de83cbe1ad7ada353/linux-sandbox/6993/debug.out
--
bazel-out/k8-opt-exec-ST-31ad040affd4/bin/external/aos/aos/network/www/_reflection_test_bundle__min_terser_binary_/_reflection_test_bundle__min_terser_binary
../aos/aos/network/www/reflection_test_bundle.js --output
../aos/aos/network/www/reflection_test_bundle__min.js --config-file
bazel-out/k8-opt/bin/external/aos/aos/network/www/_reflection_test_bundle__min.minify_options.json)
node:internal/fs/utils:356
    throw err;
    ^

Error: ENOENT: no such file or directory, lstat
'/dev/shm/bazel-sandbox.a3f391e631c5ea6c72bb0d2c0b90317c117ec497a3759d6de83cbe1ad7ada353/linux-sandbox/6993/execroot/spacecookies/bazel-out/k8-opt/aos/aos/network/www/reflection_test_bundle.js'
    at Object.lstatSync (node:fs:1666:3)
at Object.lstatSync
(/home/austin/.cache/bazel/_bazel_austin/0654a46dce625c9bc6e0b54d6df8dcff/external/aspect_rules_js/js/private/node-patches/fs.cjs:125:23)
at isDirectory
(/dev/shm/bazel-sandbox.a3f391e631c5ea6c72bb0d2c0b90317c117ec497a3759d6de83cbe1ad7ada353/linux-sandbox/6993/execroot/spacecookies/bazel-out/k8-opt-exec-ST-31ad040affd4/bin/external/aos/aos/network/www/_reflection_test_bundle__min_terser_binary_/_reflection_test_bundle__min_terser_binary.runfiles/aos/aos/network/www/_reflection_test_bundle__min_terser_runner.cjs:25:13)
    at Array.find (<anonymous>)
at main
(/dev/shm/bazel-sandbox.a3f391e631c5ea6c72bb0d2c0b90317c117ec497a3759d6de83cbe1ad7ada353/linux-sandbox/6993/execroot/spacecookies/bazel-out/k8-opt-exec-ST-31ad040affd4/bin/external/aos/aos/network/www/_reflection_test_bundle__min_terser_binary_/_reflection_test_bundle__min_terser_binary.runfiles/aos/aos/network/www/_reflection_test_bundle__min_terser_runner.cjs:251:15)
at Object.<anonymous>
(/dev/shm/bazel-sandbox.a3f391e631c5ea6c72bb0d2c0b90317c117ec497a3759d6de83cbe1ad7ada353/linux-sandbox/6993/execroot/spacecookies/bazel-out/k8-opt-exec-ST-31ad040affd4/bin/external/aos/aos/network/www/_reflection_test_bundle__min_terser_binary_/_reflection_test_bundle__min_terser_binary.runfiles/aos/aos/network/www/_reflection_test_bundle__min_terser_runner.cjs:269:3)
    at Module._compile (node:internal/modules/cjs/loader:1364:14)
    at Module._extensions..js (node:internal/modules/cjs/loader:1422:10)
    at Module.load (node:internal/modules/cjs/loader:1203:32)
    at Module._load (node:internal/modules/cjs/loader:1019:12) {
  errno: -2,
  syscall: 'lstat',
  code: 'ENOENT',
path:
'/dev/shm/bazel-sandbox.a3f391e631c5ea6c72bb0d2c0b90317c117ec497a3759d6de83cbe1ad7ada353/linux-sandbox/6993/execroot/spacecookies/bazel-out/k8-opt/aos/aos/network/www/reflection_test_bundle.js'
}

Node.js v18.20.3

When built locally, instead of
`../aos/aos/network/www/reflection_test_bundle.js`, it references
`aos/network/www/reflection_test_bundle.js` which works. Manually
adjusting it to `external/aos/aos/network/www/reflection_test_bundle.js`
makes it build. Really, this is an abuse of short_path, so abuse it a
bit more.

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---

### Changes are visible to end-users: yes/no

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- Searched for relevant documentation and updated as needed: yes/no
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### Test plan

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---------

Signed-off-by: Austin Schuh <[email protected]>

Author: AustinSchuh

terser/private/terser.bzl

@@ -30,6 +30,11 @@ _ATTRS = {
 def _filter_js(files):
     return [f for f in files if f.is_directory or f.extension == "js" or f.extension == "mjs"]
 
+def maybe_prefix_external(path):
+    if path.startswith("../"):
+        return "external" + path[2:]
+    return path
+
 def _impl(ctx):
     args = ctx.actions.args()
 
@@ -49,8 +54,8 @@ def _impl(ctx):
         if ctx.attr.sourcemap:
             output_sources.append(ctx.actions.declare_file("%s.js.map" % ctx.label.name))
 
-    args.add_all([s.short_path for s in input_sources])
-    args.add_all(["--output", output_sources[0].short_path])
+    args.add_all([maybe_prefix_external(s.short_path) for s in input_sources])
+    args.add_all(["--output", maybe_prefix_external(output_sources[0].short_path)])
 
     debug = ctx.attr.debug or ctx.var["COMPILATION_MODE"] == "dbg"
     if debug:
@@ -90,7 +95,7 @@ def _impl(ctx):
                 "\"bazel_no_debug\"": str(not debug).lower(),
             },
         )
-        args.add_all(["--config-file", options.short_path])
+        args.add_all(["--config-file", maybe_prefix_external(options.short_path)])
 
     ctx.actions.run(
         inputs = inputs,