Add optional `path-mapping` warning for path strings in action command lines

Bazel's path mapping (https://github.com/bazelbuild/bazel/discussions/22658)
can only rewrite `File` objects passed to `ctx.actions.args()`. Path strings
obtained from `File.path`, `File.dirname`, `File.root.path`, `ctx.bin_dir.path`
or `ctx.genfiles_dir.path` are passed through unchanged and break actions that
opt into path mapping.

The new warning reports such path strings in the arguments of `args.add()`,
`args.add_all()` and `args.add_joined()` as well as in the `executable`,
`arguments` and `command` parameters of `ctx.actions.run()` and
`ctx.actions.run_shell()`. It is disabled by default and can be enabled with
`--warnings=+path-mapping`.