build: replace SourceName with PKG_NAME compile unit name

build: replace SourceName with PKG_NAME compile unit name

Currently, the same information is stored at the Packages.manifest in
the 'Package:' variable and also additionally in the 'SourceName:' variable.

So we have for Packages.manifest for strongswan-charon-cmd:
```
Package: strongswan-charon-cmd
Version: 5.9.11-1

SourceName: strongswan-charon-cmd
License: GPL-2.0-or-later
Section: net
```

This is not correct. Several installable packages are built from the same
strongswan source. Therefore it makes more sense that the source name is
really the source name. In this case the it is 'strongswan'.

After this change the Packages.manifest for strongswan-charon-cmd:
```
Package: strongswan-charon-cmd
Version: 5.9.11-1

SourceName: strongswan
License: GPL-2.0-or-later
Section: net
```

In summary. The 'Package' name is the name of the package to be installed
on the target system. The 'SourceName' is the compile unit from which the
package was build from. This must be the same for all installable
packages built from the same compile unit. This commit fixes that.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>