revert: "fix(@ngtools/webpack): improve fallbacking for path mapping"
revert: "fix(@ngtools/webpack): improve fallbacking for path mapping"

This reverts commit 47f937e28f0a9d82997bd29630a8b3b92f44c0f5.

This commit had a few issues detected by the NativeScript project. It is
likely that they would have affected more users (and libraries) and so
it is safer to revert now and investigate later. Will likely be fixed in
6.2 by refactoring this whole plugin properly.

beeman/angular-cli,beeman/angular-cli,catull/angular-cli,manekinekko/angular-cli,filipesilva/angular-cli,manekinekko/angular-cli,catull/angular-cli,DevIntent/angular-cli,hansl/angular-cli,ocombe/angular-cli,catull/angular-cli,manekinekko/angular-cli,geofffilippi/angular-cli,Brocco/angular-cli,Brocco/angular-cli,ocombe/angular-cli,beeman/angular-cli,geofffilippi/angular-cli,angular/angular-cli,clydin/angular-cli,catull/angular-cli,DevIntent/angular-cli,hansl/angular-cli,angular/angular-cli,Brocco/angular-cli,DevIntent/angular-cli,Brocco/angular-cli,Brocco/angular-cli,angular/angular-cli,ocombe/angular-cli,filipesilva/angular-cli,DevIntent/angular-cli,hansl/angular-cli,geofffilippi/angular-cli,angular/angular-cli,hansl/angular-cli,geofffilippi/angular-cli,clydin/angular-cli,clydin/angular-cli,ocombe/angular-cli,filipesilva/angular-cli,filipesilva/angular-cli,beeman/angular-cli,ocombe/angular-cli,clydin/angular-cli,DevIntent/angular-cli