Make `path-mapping` messages specific and fix simple cases automatically

Each kind of path string (`File.path`, `File.dirname`, `File.root.path`,
`ctx.bin_dir.path`) now gets its own message tailored to where it is used.

Simple cases are fixed automatically: a `File.path` passed to `args.add()`,
`args.add_all()`, `args.add_joined()` or `executable`, optionally formatted
into a string literal or wrapped in a list comprehension, is replaced by the
`File` object and a `format` or `format_each` parameter. Variables created
with `ctx.actions.declare_directory()` are passed with
`expand_directories = False` since `args.add()` rejects directories.