Make handling of package configuration files consistent with handling of package curation files

[mnonnenmacher]

Align the handling of package configuration files with the handling of package curation files for consistency. Current differences:

• Package configurations and curations can both be stored in a single file or in a multiple files inside a directory. By default, package curations are read from both, the default file and directory locations. Package configurations are only read from the directory by default.
• The CLI options for curations use plural, for example --package-curations-file. The options for configurations use singular, for example --package-configuration-file.

Also see the discussion in #6497.

[sschuberth]

The options for configurations use singular

And that's probably because we actually use readValue<PackageConfiguration>() pretty much everywhere except for FilePackageConfigurationProvider where we use readValue<List<PackageConfiguration>>(). Which means our code is not only inconsistent in the CLI option naming, but also with regard to whether we expect a single package configuration file to contain single or multiple package configurations. It should be the latter to align with package curations.

[sschuberth]

This has been partly addressed in https://github.com/oss-review-toolkit/ort/pull/6858.