rebar_dir: Avoid canonicalizing the CWD (get_cwd/0)

[the-mikedavis]

The unix BIF which ultimately implements file:get_cwd/0 uses getcwd(3) which returns a canonicalized path, resolving any symlinks. Instead we should try to preserve the non-canonicalized path by reading the $PWD environment variable set by Posix shells. We should prefer $PWD to file:get_cwd() when it canonicalizes to the same path. This emulates the behavior of pwd -L (the default behavior of pwd).

WhatsApp/erlang-language-platform#65 has more details on the motivation of this change. On my machine rebar3 experimental manifest emits canonicalized paths which confuse tools like ELP that comumnicate these paths with editors.

[the-mikedavis]

Looks like this will need some more work, it breaks rebar3 eunit for me locally

[ferd]

I think this is going to be a bigger changeset than you intend, because we make extensive use of filename:absname/1 in code:

• In path variables used by external commands and possibly the C compiler
• in fetching dependencies
• setting up the base directory for Rebar3's own operations for all build artifacts (including compiled modules, but also code coverage), across multiple compilers and most test tools
• in building escripts
• in executing custom scripts in rebar3 shell
• in path name canonicalization and retargetting, which may be used by plugins.

The definition of that function, however, calls to file:get_cwd() internally, which currently does not respect the behavior intended by this PR.

I would expect that inconsistent path expansion here is likely to cause some problems or bypass the fixes? We do rely on absolute paths by default, a whole lot, everywhere.

We'd probably need to supply our own absname function that does not do the symlink following, and then substitute it everywhere.