With this change, it's much easier for bazel projects to depend on
Catch. They just need to add:
- In the workspace:
```
http_archive(
name = "com_github_catchorg_catch2",
urls = ["https://github.com/catchorg/Catch2/archive/v2.12.1.tar.gz"],
strip_prefix = "Catch2-2.12.1",
sha256 = "e5635c082282ea518a8dd7ee89796c8026af8ea9068cd7402fb1615deacd91c3",
)
```
Or the appropriate version/sha256.
- For the tests, assuming that `test_main.cc` contains the
`CATCH_CONFIG_MAIN`:
```
cc_library(
name = "test_main",
srcs = ["test_main.cc"],
deps = ["@com_github_catchorg_catch2//:catch2"],
)
```
