Take into account repository mapping when processing labels inside BUILD files within external repositories.

For example:
a/BUILD
genrule(
  name = "a",
  srcs = ["@x//:x.txt"],
  outs = ["result.txt"],
  cmd = "echo hello > \$(location result.txt)"
)

If the main workspace file references that repository with a rule:
local_repository(
  name = "other_repo",
  path = "../a",
  repo_mapping = {"@x" : "@y"}
)

Then when a/BUILD is evaluated, the string "@x//:x.txt" will be turned into a Label "@y//:x.txt"

RELNOTES: None
PiperOrigin-RevId: 201562148