Add support-path-mapping to mojo_library (#36)

The only changes here are making sure we let the args object handle path
mapping. This is slightly complicated by using the dirname for import
paths, but ignoring the value from our provider.

Author: keith

mojo/mojo_library.bzl

@@ -3,14 +3,17 @@
 load("//mojo:providers.bzl", "MojoInfo")
 load("//mojo/private:utils.bzl", "MOJO_EXTENSIONS", "collect_mojoinfo")
 
+def _format_include(arg):
+    return ["-I", arg.dirname]
+
 def _mojo_library_implementation(ctx):
     mojo_toolchain = ctx.toolchains["//:toolchain_type"].mojo_toolchain_info
 
     mojo_package = ctx.actions.declare_file(ctx.label.name + ".mojopkg")
     args = ctx.actions.args()
     args.add("package")
     args.add("-strip-file-prefix=.")
-    args.add("-o", mojo_package.path)
+    args.add("-o", mojo_package)
     args.add_all(ctx.attr.copts)
 
     import_paths, transitive_mojopkgs = collect_mojoinfo(ctx.attr.deps + mojo_toolchain.implicit_deps)
@@ -19,9 +22,9 @@ def _mojo_library_implementation(ctx):
     file_args = ctx.actions.args()
     for file in ctx.files.srcs:
         if not file.dirname.startswith(root_directory):
-            file_args.add("-I", file.dirname)
+            args.add_all([file], map_each = _format_include)
 
-    file_args.add_all(import_paths, before_each = "-I")
+    file_args.add_all(transitive_mojopkgs, map_each = _format_include)
     file_args.add(root_directory)
     ctx.actions.run(
         executable = mojo_toolchain.mojo,
@@ -37,6 +40,9 @@ def _mojo_library_implementation(ctx):
         },
         use_default_shell_env = True,
         toolchain = "//:toolchain_type",
+        execution_requirements = {
+            "supports-path-mapping": "1",
+        },
     )
 
     transitive_runfiles = []