link_overwrite: allow to overwrite files from alias
link_overwrite: allow to overwrite files from alias

See https://github.com/Homebrew/homebrew-core/pull/54912#issuecomment-653893645
Formulary.factory("python") points to python@3.8, which breaks link_overwrite
for that case.

This changes checks if the formula is an alias, so that we can still override
the files during installation.

claui/brew,claui/brew,JCount/brew,mahori/brew,sjackman/homebrew,Homebrew/brew,nandub/brew,Homebrew/brew,MikeMcQuaid/brew,JCount/brew,JCount/brew,Homebrew/brew,nandub/brew,konqui/brew,sjackman/homebrew,MikeMcQuaid/brew,konqui/brew,nandub/brew,nandub/brew,mahori/brew,Homebrew/brew,EricFromCanada/brew,sjackman/homebrew,vitorgalvao/brew,claui/brew,reitermarkus/brew,reitermarkus/brew,konqui/brew,vitorgalvao/brew,JCount/brew,MikeMcQuaid/brew,MikeMcQuaid/brew,EricFromCanada/brew,vitorgalvao/brew,sjackman/homebrew,vitorgalvao/brew,konqui/brew,mahori/brew,mahori/brew,EricFromCanada/brew,claui/brew,reitermarkus/brew,EricFromCanada/brew,reitermarkus/brew