Running ament_clang_tidy results in Could not find 'clang-tidy-6.0' executable

Hello,

I would like to start using clang-tidy in all my ROS2 packages.

I am using the OSRF supplied ROS2 galactic docker image that comes with clang-tidy
```
root@rfriedm-us-dl01:~/tas-ros2-system# clang-tidy --version
LLVM (http://llvm.org/):
LLVM version 10.0.0

Optimized build.
Default target: x86_64-pc-linux-gnu
Host CPU: skylake-avx512
```

Docs [recommends](https://github.com/ament/ament_lint/blob/galactic/ament_clang_tidy/doc/index.rst#how-to-run-the-check-from-the-command-line) to use clang-tidy-6.0. but I don't have that version. I can install it directly with
```
apt install clang-tidy-6.0
```

I have built my package according to documentation.
```
colcon build --packages-up-to tas_tn_002 --mixin compile-commands --cmake-args -DBUILD_TESTING=ON
```

Now, I can run ament_clang_tidy successfully.
```
ament_clang_tidy build
```

If I don't go out and install `ament-clang-tidy-6.0`, I get this error:
```
Could not find 'clang-tidy-6.0' executable
```

Now, if I try using the [cmake integration](https://github.com/ament/ament_lint/blob/galactic/ament_cmake_clang_tidy/doc/index.rst),with the appropriate calls `package.xml` and `CMakeLists.txt`, rosdep still doesn't install the depenency.

```
rosdep install -i path/to/tas_tn_002
...
```

1. Is there a reason it is tied to clang-tidy-6, or can you support the version installed by OSRF for galactic?
2. If version 6.0 is required, should I submit a PR with the command to install it?
3. If I am using rosdep to install dependencies, why does it not install clang-tidy-6.0 as a dependency?