fix(@angular/cli): inform user and error if schematics package is in unreachable location (#16466)
fix(@angular/cli): inform user and error if schematics package is in unreachable location (#16466)

This is a workaround for the schematics runtime to support the requirement of packages containing migrations (or other schematics) to not have a direct/runtime dependency on the schematics package.

Closes: #16392
filipesilva/angular-cli,angular/angular-cli,filipesilva/angular-cli,catull/angular-cli,hansl/angular-cli,ocombe/angular-cli,geofffilippi/angular-cli,angular/angular-cli,filipesilva/angular-cli,Brocco/angular-cli,hansl/angular-cli,manekinekko/angular-cli,ocombe/angular-cli,geofffilippi/angular-cli,catull/angular-cli,beeman/angular-cli,clydin/angular-cli,ocombe/angular-cli,filipesilva/angular-cli,clydin/angular-cli,catull/angular-cli,clydin/angular-cli,hansl/angular-cli,clydin/angular-cli,ocombe/angular-cli,geofffilippi/angular-cli,beeman/angular-cli,angular/angular-cli,Brocco/angular-cli,Brocco/angular-cli,Brocco/angular-cli,hansl/angular-cli,geofffilippi/angular-cli,ocombe/angular-cli,beeman/angular-cli,manekinekko/angular-cli,manekinekko/angular-cli,catull/angular-cli,angular/angular-cli,Brocco/angular-cli,beeman/angular-cli