Do not list bst sources

for example:

cc_library(
    name = "bazelize-makelib1",
    srcs = ['makelib1', 'usr/testlibs1/lib1.so', 'usr/testlibs1/lib2.a', 'usr/testlibs1/lib3.so.1234.4321', 'usr/testlibs1/lib4.pic.a', 'usr/testlibs1/lib5.a', 'usr/testlibs1/lib6.pic.lo', 'usr/testlibs1/lib7.lo'],
    hdrs = ['usr/testincludes1/hdr1.h', 'usr/testincludes1/hdr2.hh', 'usr/testincludes1/hdr3.hpp', 'usr/testincludes1/hdr4.hxx', 'usr/testincludes1/hdr5.inc', 'usr/testincludes1/hdr6.inl', 'usr/testincludes1/hdr7.H'],
)

leads to: in srcs attribute of cc_library rule //test:bazelize-makelib1: target '//test:makelib1' does not exist.

that path does not exist in the artifact checkout and is interpreted as an undefined label

bst 'sources' are not the same as 'srcs' values for the rules we've covered and should not be collected from the element.sources() but only from the manifest