language/python: add package_available? helper
language/python: add package_available? helper

Add helper to check whether a module is importable.

Closes Homebrew/homebrew#37345.

Signed-off-by: Tim D. Smith <46f1a0bd5592a2f9244ca321b129902a06b53e03@tim-smith.us>

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