This commit adds two alpha flags to `opm render` and updates
FBC library code to support efficient filtering of FBC:
- `--alpha-filter-config=./path/to/config`
- `--alpha-keep-packages=pkg1,pkg2`

To support the filter config option, this commits defines a new
configuration file format that supports:
- filtering by package
- within a package, filtering by channel
- within a package, selecting a new default channel (required if the original default
  channel is filtered out)
- within each channel, filtering by a version range

In order to support correct version range filtering, some bundles
that fall outside the version range may remain in the channel to
ensure that the filtered channel is still valid and includes all
bundles that did fall in the range

Included in this commit is an update to declcfg.WalkMetasFS to
enable concurrency, similar to recent changes in declcfg.LoadFS.
declcfg.LoadFS is also refactored internally to use the new
declcfg.WalkMetasFS implementation.

Signed-off-by: Joe Lanford <joe.lanford@gmail.com>

Signed-off-by: Joe Lanford <joe.lanford@gmail.com>