enforce absolute paths as start dir of extensions

[lexming]

Currently, self.start_dir in regular packages is always an absolute path. So it can be easily used in easyblocks regardles of cwd in the same way as self.installdir and self.builddir.

However, self.start_dir of extensions is either None or the path set in self.cfg['start_dir'] of the extension, which is always a relative path.

This PR updates ExtensionEasyBlock._set_start_dir to always define self.start_dir as an absolute path. Standardizing its form across packages and extensions. The resulting start dir will be set according to the following preference order:

1. if self.start_dir is an absolute path and exists it will be used as is
2. combine self.ext_dir with self.start_dir

⚠️ Breaking changes:

• breaks RPackages as it prepends stuff to self.start_dir:
  fixed in use start dir of extension to install R packages easybuild-easyblocks#2867

[ocaisa]

LGTM