memoize `./pants path` lookups

it was dumbly not re-using previous work
memoize `pants paths`

split out path and paths implementations; they need to work
differently to each be as fast as possible
Missing imports
Fix bug in exclusives.

- IvyResolve was erroneously using the wrong key for a target group with no declared
  exclusives.

Auditors: benjy
Remove debugging print. (Sorry about that)

Auditors: benjy
Exclusives bugfix.

Builds with no exclusives would sometimes fail due to a bug
in how execution order was planned.

Auditors: benjy
Remove old classpath code.

- Now that all code is using the new exclusives-based classpath
  mechanism, get rid of the old one.

Auditors: benjy
Merge pull request #149 from orsenthil/master

Fix a typo in pex.py
Tweak exclusives error message.

Also tweak goal name.
Missing imports
Fix bug in exclusives.

- IvyResolve was erroneously using the wrong key for a target group with no declared
  exclusives.

Auditors: benjy
Remove debugging print. (Sorry about that)

Auditors: benjy
Exclusives bugfix.

Builds with no exclusives would sometimes fail due to a bug
in how execution order was planned.

Auditors: benjy
Remove old classpath code.

- Now that all code is using the new exclusives-based classpath
  mechanism, get rid of the old one.

Auditors: benjy
Tweak exclusives error message.

Also tweak goal name.
Merge remote-tracking branch 'twitter/master'
Fix "locahost" typo
Merge pull request #152 from danielatdattrixdotcom/patch-1

Fix "locahost" typo
Clarify Phase.execute and make sure it has a run_tracker.
Merge remote-tracking branch 'twitter/master'