scripts/package-metadata.pl: inhibit compile deps on missing build types

scripts/package-metadata.pl: inhibit compile deps on missing build types

When a package declares a PKG_BUILD_DEPENDENCY or HOST_BUILD_DEPENDENCY on
a not existing build type, the metadata script will emit a reference to an
unresolvable build target in tmp/.packagedeps, causing the make process to
fail hard in a way not catchable by the IGNORE_ERRORS mechanism.

In a situation where a package "test-a" declares a build dependency
"PKG_BUILD_DEPENDS:=test-b/host" while the Makefile of "test-b" does not
implement a HostBuild, make fails with an unrecoverable error in the form:

    make[1]: Entering directory '...'
    make[1]: *** No rule to make target 'package/test-b/host/compile',
                 needed by 'package/test-a/compile'.  Stop.
    make[1]: Leaving directory '...'
    .../toplevel.mk:200: recipe for target 'package/test-a/compile' failed
    make: *** [package/test-a/compile] Error 2

Extend the metadata generation script to catch such unresolved references
and emit a visable warning upon detection.

After this change, the script will emit a warning similar to:

    WARNING: Makefile "package/test-a/Makefile" has a build dependency on
    "test-b/host" but "package/test-b/Makefile" does not implement a
    "host" build type

Fixes a global build cluster outage which occured after the "python-cffi"
feed package removed its HostBuild which the "python-cryptography" package
build-depended on.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>