Separate the loading of repo definitions from JSON file from
`LoadRepositories()` to a standalone function
`loadRepositoriesFromFile()`, to make it easy to reuse it in the future.
Add unit tests for `LoadRepositories()` function.
Exclude github.com/osbuild/osbuild-composer/internal/rpmmd/test package
from test coverage. Package with just tests and no other code makes `go
test` to fail. This should be fixed in go 1.17.
See https://github.com/golang/go/issues/27333
Signed-off-by: Tomas Hozza <thozza@redhat.com>