Search for files named Blueprints only under build/blueprint

to enable other subprojects to use their own instance of
Blueprint in other directories.

Files named Android.bp are still autodetected throughout the
tree like previously.

Bug: 64363847
Test: mkdir -p subdir \
      && echo "syntax error" > Blueprints \
      && m -j nothing

Test: build/soong/scripts/diff_build_graphs.sh \
      --products=aosp_arm \
      'build/soong:work^' 'build/soong:work'

Change-Id: I8199f12b68dc1699bf44682b86169def37b53a5e