Fix building packages by provides name (#1303)

Use case: `mkbootimg` provides the `unpackbootimg` package. When
running `pmb.chroot.apk.install(args,"unpackbootimg")`, it was not
able to properly build the package.

Reproducing the error:
```
sudo rm ~/.local/var/pmbootstrap/packages/x86_64/mkbootimg*
pmbootstrap index
pmbootstrap --mirror-pmOS="" chroot --add=unpackbootimg
```

Or alternatively (simpler but less illustrative):
```
pmbootstrap build unpackbootimg --force
```