bp2build: allowlist //external/libcap/...

This builds cap_names.list.h, which uses an eponymous filegroup
"generate_cap_names_list.awk" in Soong, but uses the file target
directly in Bazel.

This also improve filegroup support for mixed builds, by issuing a
cquery call _without_ arch. Filegroups in Soong don't have configurable
properties, so don't generate Bazel filegroups into buildroot's
config_nodes (which was x86_64 by default).

The mixed_build_root now looks like this:

```
config_node(...)
config_node(...)
config_node(...)
config_node(...)

...

filegroup(name = "common",
    srcs = ["@//bionic/linker:linker_sources_x86",
            "@//bionic/libc:kernel_input_headers",
            "@//system/timezone/apex:com.android.tzdata-androidManifest",
            "@//external/libcap:generate_cap_names_list.awk",
            "@//bionic/linker:linker_sources_arm64",
            "@//bionic/linker:linker_sources",
            "@//bionic/libc:libc_sources_shared_arm",
            "@//bionic/linker:linker_sources_x86_64",
            "@//bionic/libc:all_kernel_uapi_headers",
            "@//build/bazel/examples/apex/minimal:build.bazel.examples.apex.minimal-file_contexts",
            "@//system/core/libcutils:android_filesystem_config_header",
            "@//bionic/libc:libc_sources_static",
            "@//bionic/linker:linker_sources_arm",
            "@//bionic/libc/tools:bionic-gensyscalls",
            "@//bionic/tools:bionic-generate-version-script",
            "@//bionic/libc:libc_sources_shared"],
)

mixed_build_root(name = "buildroot",
    deps = [":x86",
            ":arm64",
            ":arm",
            ":common",
            ":x86_64"],
)
```

Test: CI
Fixes: 198595323
Fixes: 198235838
Change-Id: I6df9a14da556cf358d96e6a99b514f66a2638295