REBAR_SRC_DIRS and REBAR_APP_DIRS fix by making rebar_dir:src_dirs/1 options default to ["src"]

[MarkoMin]

["src"] becomes the default for src_dirs. It was de facto standard because all calls to rebar_dir:src_dirs/2 were using it as a default.

Both REBAR_APP_DIRS and REBAR_SRC_DIRS are relative now. Note that those were undocumented for now so I think it's appropriate to change their values. REBAR_APP_DIRS previously pointed to non-existing directories anyways, same as REBAR_SRC_DIRS.

Changed the testcase to test whether all env variables are exported.

Closes #2869

[ferd]

Two tiny changes to sort of make sure we don't accidentally drop support for empty option sets, and then it's good to merge.

[ferd]

oh if the rebar_dir:all_src_dirs/2 call no longer exists, I think we should bring it back. We don't know if plugins use it, in which case the plugins would stop working entirely.

misread the commit message's meaning