hammer.git: allow overriding package content

Allow overriding package source code content by providing a local
directory with a source tree. The directory is passed in an environment
variable "<pkg>_sources", e.g.

	gcc_sources=$HOME/git/gcc make toolchain

would build a toolchain using gcc sources from a local directory
regardless what the current package tar ball or remote git contains.
This can be useful when e.g. bisecting issues caused by a package
upgrade or when developing patches against a package.