caveats: highlight PKG_CONFIG_PATH if exists
caveats: highlight PKG_CONFIG_PATH if exists

We keep getting issues every now & again about people being confused that
keg_only includes hiding the `pkg-config` files. Perhaps seems reasonable to
start pointing out those exist when they do & a hint on how to access them.

Hidden behind a which check to avoid redundant messages, although `pkg-config`
is an incredibly popular formula. Ran some time tests, no appreciable difference
from the status quo.

If accepted:
Closes https://github.com/Homebrew/homebrew-core/issues/4669.

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