Previously we always walked the whole folder tree looking for
entry-points before we tested whether a target package had been
processed already. This could take >10secs!
This commit does a quick check of the target package before doing
the full walk which brings down the execution time for ngcc in this
case dramatically.
```
$ time ./node_modules/.bin/ivy-ngcc -t @angular/common/http/testing
Compiling @angular/core : fesm2015 as esm2015
Compiling @angular/core : fesm5 as esm5
Compiling @angular/core : esm2015 as esm2015
Compiling @angular/core : esm5 as esm5
Compiling @angular/common/http : fesm2015 as esm2015
Compiling @angular/common/http : fesm5 as esm5
Compiling @angular/common/http : esm2015 as esm2015
Compiling @angular/common/http : esm5 as esm5
Compiling @angular/common/http/testing : fesm2015 as esm2015
Compiling @angular/common/http/testing : fesm5 as esm5
Compiling @angular/common/http/testing : esm2015 as esm2015
Compiling @angular/common/http/testing : esm5 as esm5
real 0m19.766s
user 0m28.533s
sys 0m2.262s
```
```
$ time ./node_modules/.bin/ivy-ngcc -t @angular/common/http/testing
The target entry-point has already been processed
real 0m0.666s
user 0m0.605s
sys 0m0.113s
```
PR Close#29740