Fix to use $crate in path macros

[bouzuya]

error[E0433]: failed to resolve: could not find `path` in `struct_path`
 --> tests/path_test.rs:7:16
  |
7 |     assert_eq!(firestore::path!(MyTestStructure::some_id), "some_id");
  |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ could not find `path` in `struct_path`
  |
  = note: this error originates in the macro `firestore::path` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0433]: failed to resolve: could not find `paths` in `struct_path`
 --> tests/path_test.rs:9:9
  |
9 |         firestore::paths!(MyTestStructure::{some_id, some_num}),
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ could not find `paths` in `struct_path`
  |
  = note: this error originates in the macro `firestore::paths` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0433]: failed to resolve: could not find `path` in `struct_path`
  --> tests/path_test.rs:13:9
   |
13 |         firestore::path_camel_case!(MyTestStructure::some_id),
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ could not find `path` in `struct_path`
   |
   = note: this error originates in the macro `firestore::path_camel_case` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0433]: failed to resolve: could not find `paths` in `struct_path`
  --> tests/path_test.rs:17:9
   |
17 |         firestore::paths_camel_case!(MyTestStructure::{some_id, some_num}),
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ could not find `paths` in `struct_path`
   |
   = note: this error originates in the macro `firestore::paths_camel_case` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0433`.
error: could not compile `firestore` (test "path_test") due to 4 previous errors

 *  The terminal process "cargo 'test', '--package', 'firestore', '--test', 'path_test', '--', 'test_ambiguous_path_macro', '--exact', '--nocapture'" terminated with exit code: 101.
 *  Terminal will be reused by tasks, press any key to close it.

FYI: https://doc.rust-lang.org/reference/macros-by-example.html#metavariables

[abdolence]

Hey, thanks for your contributions and especially appreciated for tests (!)

[abdolence]

LGTM