scripts/globals: add support for "internal files".

Internal files are files which are stored in separate per-file
namespaces and loaded on demand.
packages/pkgbuild: add unmodified makepkg source (version 4.1.0) as an internal file for pkgbuild package.

This will be used to offload some parts of package processing, mostly
related to pacman 4.1 sophisticated VCS support.
scripts/globals: fix internal file support: source file name is BASH_SOURCE, not FUNCNAME.
scripts/globals: fix internal file support: we use directory "internal".
packages/makepkg: internal pkgbuild: prepare the file for library-like usage.

Remove unwanted parts of "main" code and make check_build_status a stub.
packages/pkgbuild: pkgbuild_version: add epoch support.

(copied from makepkg code)
packages/pkgbuild: improve parameter passing in _pkgbuild_vcs_process.

Use (existing) bash local variables instead of positional arguments
to pass some data.
packages/pkgbuild: add top-level support for new-style VCS repositories.

- add library functions _pkgbuild_vcs_get_repo_name and
_pkgbuild_vcs_match_repo_type
- source the makepkg internal file to parse the PKGBUILD
and prepare the sources
- use "$srcdir" instead of "$DIR/src"
- finally, iterate over source array and find VCS elements to process