Create a way for macros to "reify" string labels into label objects

[Wyverald]

The immediately obvious answer seems to be "just call Label(...)", except that would anchor the string label at the current .bzl file's context. For a macro that receives string labels from BUILD files, it would probably want to convert string labels with the BUILD file's context. Example:

# @myproject//pkg/BUILD
load("@myrule//myrule:defs.bzl", "mymacro")
mymacro(
    name = "mytarget",
    deps = [":hello"],  # Intended to be @myproject//pkg:hello
)
filegroup(name = "hello")


# @myrule//myrule/defs.bzl
DEFAULT_DEP = Label("//myrule:compiler")  # Always points to @myrule//myrule:compiler, regardless of repo mappings
def mymacro(name, deps):
    if DEFAULT_DEP not in deps:  # THIS LINE DOES NOT WORK
        deps += [DEFAULT_DEP]
    myrule(name, deps)

Here mymacro simply includes an extra dep before forwarding the call to myrule. But the check is almost never true because deps almost always contains only strings! We can't do [Label(dep) for dep in deps] because that would mean ":hello" gets turned into @myrule//myrule:hello instead of @myproject//pkg:hello. We also can't check if str(DEFAULT_DEP) not in deps because deps can contain non-absolute labels, whereas str(DEFAULT_DEP) is an absolute label.

We need a way to turn a string into a Label using the calling package's context. This is actually exactly the deprecated Label(..., relative_to_caller_package=True) constructor that we removed, thinking nobody would ever use it. It turns out that's not true (see protocolbuffers/protobuf#9688). We should maybe add it back, and maybe think of a better API for it.

cc @brandjon @meteorcloudy

[pcjanzen]

does
Label(native.repository_name() + "//" + native.package_name()).relative(dep)
solve this problem?
(since native.repository_name() and native.package_name() are relative to the caller package?)

[Wyverald]

That's a nice trick! Unfortunately it breaks when repo mappings are involved. Label(...) resolves the text using the current .bzl file's repo mapping, but native.repository_name() is the "canonical" repo name (post-mapping).

However, this idea could still be valuable if we decide to fix #14580 by allowing some sort of "canonical-repo-name absolute label literals".

[Wyverald]

Duping against #15593