Refactor fetch
Refactor fetch

Remove direct references to checksum types, access them through the
Checksum::TYPES constant instead.

AnastasiaSulyagina/brew,mistydemeo/homebrew,sjackman/homebrew,bfontaine/brew,mgrimes/brew,mistydemeo/homebrew,sjackman/homebrew,palxex/brew,konqui/brew,muellermartin/dist,claui/brew,mistydemeo/homebrew,AnastasiaSulyagina/brew,toonetown/homebrew,mahori/brew,vitorgalvao/brew,konqui/brew,palxex/brew,gordonmcshane/homebrew,bfontaine/brew,toonetown/homebrew,vitorgalvao/brew,ilovezfs/brew,pseudocody/brew,gregory-nisbet/brew,Homebrew/brew,MikeMcQuaid/brew,rwhogg/brew,ilovezfs/brew,mgrimes/brew,reitermarkus/brew,hanxue/linuxbrew,amar-laksh/brew_sudo,mahori/brew,aw1621107/brew,Linuxbrew/brew,gordonmcshane/homebrew,claui/brew,zmwangx/brew,hanxue/linuxbrew,reitermarkus/brew,nandub/brew,zmwangx/brew,Homebrew/brew,nandub/brew,reelsense/brew,claui/brew,pseudocody/brew,EricFromCanada/brew,zmwangx/brew,alyssais/brew,gregory-nisbet/brew,sjackman/homebrew,maxim-belkin/brew,hanxue/linuxbrew,vitorgalvao/brew,palxex/brew,reelsense/brew,alyssais/brew,JCount/brew,EricFromCanada/brew,Homebrew/brew,tonyg/homebrew,nandub/brew,mahori/brew,aw1621107/brew,amar-laksh/brew_sudo,JCount/brew,reelsense/brew,reitermarkus/brew,staticfloat/brew,maxim-belkin/brew,mgrimes/brew,rwhogg/brew,EricFromCanada/brew,DomT4/brew,sjackman/homebrew,AnastasiaSulyagina/brew,vitorgalvao/brew,DomT4/brew,pseudocody/brew,amar-laksh/brew_sudo,muellermartin/dist,konqui/brew,EricFromCanada/brew,gregory-nisbet/brew,nandub/brew,DomT4/brew,gordonmcshane/homebrew,DomT4/brew,Linuxbrew/brew,MikeMcQuaid/brew,jmsundar/brew,tonyg/homebrew,JCount/brew,JCount/brew,jmsundar/brew,toonetown/homebrew,jmsundar/brew,MikeMcQuaid/brew,reitermarkus/brew,tonyg/homebrew,staticfloat/brew,aw1621107/brew,rwhogg/brew,staticfloat/brew,Linuxbrew/brew,mahori/brew,konqui/brew,ilovezfs/brew,maxim-belkin/brew,Linuxbrew/brew,Homebrew/brew,muellermartin/dist,MikeMcQuaid/brew,alyssais/brew,bfontaine/brew,claui/brew