Add check for handcrafted BUILD file in bp2build

ag/20469925 added a handcrafted BUILD file (allowlisted in ag/20597986)
to internal master. ag/c/platform/build/soong/+/20583593/12/cc/bp2build.go#818 generates references to the targets in the BUILD file. However in tm-dev, the BUILD file doesn't exist while the .afdo files do.

One solution is to cherry-pick the BUILD file to tm-dev. However, tm-dev doesn't have vendor/google/build/soong/bp2build_allowlist.go to check in the BUILD file in bp2build.

This CL adds a check that the BUILD file exists to avoid the failure as in https://android-build.googleplex.com/builds/pending/P45724500/aosp_cf_x86_64_phone-userdebug/latest/view/logs/build_error.log in tm-qpr-dev-plus-aosp branch.

Bug: 253540178
Test: go test
Change-Id: I47fb853015ca230afe3cefe1d37728bf714624be
Merged-In: I47fb853015ca230afe3cefe1d37728bf714624be