program file improvements

[TwitchBronBron]

• Improves program.getFile() to support absolute and relative paths. This more closely aligns with the v1 implementation (excluding support currently for having pkg:/ prefixes for pkgPath like v1 has);
• deprecates getFileByPathAbsolute and getFileByPkgPath in favor of using getFile for both. (they still work, but typescript will show them in strikethrough)
  
• deprecates getFilesByPkgPath in favor of `getFiles (same point as above)
• internal Program assignFile and unassignFile methods to clean up that logic (aligns with v1 implementation).
• adds helper getFirstScopeForFile function to find the first scope for a file
• moves FileLink interface into interfaces since it's not really a Program-specific interface.