With a recent change in apk [1], virtual packages of the same name will
upgrade each other. Adjust abuild to this by not using the same virtual
package name for two types of dependencies.

This fixes the way crosscompilers are built in postmarketOS [2], which
is essentially the same as running this on Alpine's gcc aport:
$ cd aports/main/gcc
$ C_TARGET_ARCH=armhf CTARGET=armv6-alpine-linux-musleabihf \
  BOOTSTRAP=nobuildbase CBUILDROOT=/ abuild -r
...
>>> gcc-armhf: Installing for host:
(1/24) Upgrading .makedepends-gcc-armhf (20190714.104731 -> 20190714.104741)
(2/24) Purging binutils-armhf (2.31.1-r2)
...

[1] apk-tools.git 37fbafcd928c466c82c892a7868d686d710e5d07
    ("add: make virtual packages upgradeable (ref #9957)")
[2] https://gitlab.com/postmarketOS/pmaports/blob/master/cross/gcc-armhf/APKBUILD

Fixes: https://gitlab.alpinelinux.org/alpine/apk-tools/issues/10649

clean up empty dirs

Before this change an invalid licence= in an APKBUILD will pass `abuild
sanitycheck`. '/usr/share/spdx/license.lst' contains one licence per line.
`grep -x` will match partial lines whereas `grep -w` will only match whole
lines.

An simple demonstration is with 'GPL-3.0' which is not a valid SPDX licence
identifier. 'GPL-3.0-only' and 'GPL-3.0-or-later' are valid licences.

```
$ grep --help
BusyBox v1.30.1 (2019-04-26 06:26:16 UTC) multi-call binary.

Usage: grep [-HhnlLoqvsriwFE] [-m N] [-A/B/C N] PATTERN/-e PATTERN.../-f FILE [FILE]...

Search for PATTERN in FILEs (or stdin)

✂
        -w      Match whole words only
        -x      Match whole lines only
✂
$ grep -w -F GPL-3.0 /usr/share/spdx/license.lst
GPL-3.0-only
GPL-3.0-or-later
$ grep -x -F GPL-3.0 /usr/share/spdx/license.lst
$
```