Next to file.nix we get src-for-file.nix 
src-for-file.nix should evaluate to a flat attribute set with 
string values.
It is supposed to be imported in the main expression.
In the ideal world it can export url, hash, version.

src-for-file.nix generation is directed by 
src-info-for-file.nix.

Attributes:

src-info-for-file.nix:

downloadPage
rev (for repos)
baseName (default = unnamed-package)
sourceRegexp (default = '.*[.]tar[.].*')
choiceCommand (default = 'head -1')
versionExtractorSedScript (default = 's/.*-([0-9.]+)[.].*/\1/')
versionReferenceCreator (default = 's/-([0-9.]+)[.]/-${version}./')
mirrorSedScript (default = none)

src-for-file.nix:

advertisedUrl (its match is the check for update presence)
url
hash
version
name
