Fix #104, #54: issues with overlong shebang in deep directories.

[tgamblin]

This does several things:

• Add sbang: a script to run scripts with long shebang lines.
  • Documentation for sbang is in bin/sbang.
• Add an sbang hook that filters the bin directory after install
  and modifies any scripts wtih shebangs that are too long to use
  sbang instead.
• sbang is at the top level, so it should be runnable (not much we
  can do if spack itself is too deep for shebang)
  • sbang, when used as the interpreter, runs the second shebang
    line it finds in a script.
  • shoud fix issues with too long shebang paths.

[tgamblin]

@cyrush @karenyyng @lee218llnl @KineticTheory

[luigi-calori]

Confirm this solve the issue I had with virtualenv.
Thanks a lot

[tgamblin]

@luigi-calori: Great! I'm forgetting what your specific issue was. Is it pypa/virtualenv#596, or something else?

I think the sbang script in this PR could be useful for these:

• Shebang length exceeded in pip executable pypa/virtualenv#596
• Figure out workaround for too long shebang lines spotify/dh-virtualenv#10, Work around too long shebang lines when installing spotify/dh-virtualenv#97

[tgamblin]

Verified to work for @KineticTheory and @luigi-calori so going to merge this.

[cyrush]

@tgamblin in the spack install environment, is there a way for me to explicitly sbang-ify a script?

The auto hook works for a subset of what we want, but we do have scripts that won't appear in the bin dir.

[cyrush]

@tgamblin figured out how to use the shebang patch directly -- it was quite easy:

from spack.hooks.sbang import *
filter_shebang(my_script)

This solves a big torn in my side, thanks!

[tgamblin]

@cyrush: would it help to make this more declarative, e.g. if packages had a property like this?

shebang_scripts = ['libexec/some-other-script', 'share/pkg/some-additional-script']

where those scripts would be automatically shebangified?

[cyrush]

@tgamblin I think I am happiest importing the shebang methods and calling them directly in the install environment.

I am creating python modules that are installed directly into a spack built python -- not their spack sanctioned paths. That may be taboo, but I believe I have sane motivation for doing so.

[tgamblin]

ok if you don't mind importing them, then that is fine. We could also move them to a more convenient place, or make them available in the build env by default.