Fix jq path in :pin runnable

Problem reported on #426

The build typically takes place in the user's workspace, so using
`$(location)` (or more correctly, `$(rootpath)`) gives an undesirable
"external/" leading segment. This would be useful if the binary being
run is in the users workspace, but we know our binary is in the same
workspace defining the BUILD file.

This change also uses the runfiles helper since I think that's required
to make this work on Windows, though I only tested on Mac.

coursier.bzl

@@ -49,12 +49,11 @@ genrule(
 sh_binary(
     name = "pin",
     srcs = ["pin.sh"],
-    args = [
-      "$(location :jq-binary)",
-    ],
-    data = [
-        ":jq-binary",
-    ],
+    # Cannot use $(location)/$(rootpath) because we'll be an external repository
+    # relative to where the build is taking place
+    args = [repository_name()[1:] + "/jq"],
+    deps = ["@bazel_tools//tools/bash/runfiles"],
+    data = [":jq-binary"],
 )
 """
 

private/pin.sh

@@ -1,12 +1,22 @@
 #!/usr/bin/env bash
 
-set -euo pipefail
+# --- begin runfiles.bash initialization v2 ---
+# Copy-pasted from the Bazel Bash runfiles library v2.
+set -euo pipefail; f=bazel_tools/tools/bash/runfiles/runfiles.bash
+source "${RUNFILES_DIR:-/dev/null}/$f" 2>/dev/null || \
+  source "$(grep -sm1 "^$f " "${RUNFILES_MANIFEST_FILE:-/dev/null}" | cut -f2- -d' ')" 2>/dev/null || \
+  source "$0.runfiles/$f" 2>/dev/null || \
+  source "$(grep -sm1 "^$f " "$0.runfiles_manifest" | cut -f2- -d' ')" 2>/dev/null || \
+  source "$(grep -sm1 "^$f " "$0.exe.runfiles_manifest" | cut -f2- -d' ')" 2>/dev/null || \
+  { echo>&2 "ERROR: cannot find $f"; exit 1; }; f=; set -e
+# --- end runfiles.bash initialization v2 ---
+
 readonly maven_install_json_loc={maven_install_location}
 # This script is run as a `sh_binary`, so ensure we are in the calling workspace before running Bazel.
 readonly execution_root=$(cd "$(dirname "$maven_install_json_loc")" && bazel info execution_root)
 readonly workspace_name=$(basename "$execution_root")
 # `jq` is a platform-specific dependency with an unpredictable path.
-readonly jq=$1
+readonly jq=$(rlocation $1)
 cat <<"RULES_JVM_EXTERNAL_EOF" | "$jq" --sort-keys --indent 4 . - > $maven_install_json_loc
 {dependency_tree_json}
 RULES_JVM_EXTERNAL_EOF