feat(lib): expose internal helpers

[yajo]

A few internal helpers are actually useful for others.

$ nix eval .#lib --apply builtins.attrNames

# Before
[ "__functor" "filterPlatforms" "importDir" "mkBlueprint" "tests" "tryImport" "withPrefix" ]

# After
[ "__functor" "entriesPath" "filterPlatforms" "importDir" "importTomlFilesAt" "mkBlueprint" "mkBlueprint'" "mkEachSystem" "optionalPathAttrs" "tests" "tryImport" "withPrefix" ]

Expose them, so they can be reused:

• entriesPath: Helper to get the path to entries/files in a directory
• importTomlFilesAt: Imports TOML files from a specified directory and converts them to Nix attribute sets, with automatic name normalization
• mkBlueprint': Internal implementation of mkBlueprint that creates the complete flake structure with all outputs (packages, configurations, modules, etc.)
• mkEachSystem: Creates a function that evaluates expressions per-system, handling cross-platform builds and system-specific configurations
• optionalPathAttrs: Returns attributes from a path if it exists, otherwise returns an empty set

@moduon MT-1075

[yajo]

Maybe mkBlueprint' could be kept internal.. wdyt?

[zimbatm]

eh, why not