In some cases, a simple rm -rf is not sufficent to clean srcdir.
One such case is the new go module system, that marks everything as
read-only - thus only letting root rm -rf it without a chmod.
There is a command intended to clean them - `go clean -modcache`.
However, for that to work, GOPATH must be defined and existent.
Running chmod for all srcdir cleanups makes no sense, nor does enforcing
root, or putting global overrides just for go.

This patch allows overriding what happens on `cleanup srcdir`, by
overriding cleanup_srcdir, and allows the use of default_cleanup_srcdir.

In our go example, it might be used as such:

cleanup_srcdir() {
	go clean -modcache
	default_cleanup_srcdir
}

Avoid DRY issues, and increase consistency.

make sure that subpackages does not inherit main package's depends.

replace litteral version string with $pkgver in source url

we need to check if a given module currently is a part of core. Modules
which have a first_release may have been removed later, for example
Module::Build.

the generation of Makefile is comparable with running configure, which
we normally do in the build() function, not in prepare.

also fix some whitespace damamge.