Fix circular import issue.
Fix circular import issue.

This cropped up when trying to use import bwa outside of the main bcbio scritps:

>>> import bcbio.ngsalign.bwa as bwa
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/n/data1/cores/bcbio/rory_kirchner/dev/bcbio-nextgen/bcbio/ngsalign/bwa.py", line 18, in <module>
    from bcbio.heterogeneity import chromhacks
  File "/n/data1/cores/bcbio/rory_kirchner/dev/bcbio-nextgen/bcbio/heterogeneity/__init__.py", line 13, in <module>
    from bcbio.heterogeneity import bubbletree, phylowgs, theta
  File "/n/data1/cores/bcbio/rory_kirchner/dev/bcbio-nextgen/bcbio/heterogeneity/bubbletree.py", line 23, in <module>
    from bcbio.structural import shared
  File "/n/data1/cores/bcbio/rory_kirchner/dev/bcbio-nextgen/bcbio/structural/__init__.py", line 13, in <module>
    from bcbio.structural import (battenberg, cn_mops, cnvkit, delly, gatkcnv, gridss,
  File "/n/data1/cores/bcbio/rory_kirchner/dev/bcbio-nextgen/bcbio/structural/battenberg.py", line 14, in <module>
    from bcbio.variation import population, vcfutils
  File "/n/data1/cores/bcbio/rory_kirchner/dev/bcbio-nextgen/bcbio/variation/population.py", line 12, in <module>
    from bcbio import install, utils
  File "/n/data1/cores/bcbio/rory_kirchner/dev/bcbio-nextgen/bcbio/install.py", line 28, in <module>
    from bcbio.pipeline import genome, version
  File "/n/data1/cores/bcbio/rory_kirchner/dev/bcbio-nextgen/bcbio/pipeline/genome.py", line 18, in <module>
    from bcbio.pipeline import alignment
  File "/n/data1/cores/bcbio/rory_kirchner/dev/bcbio-nextgen/bcbio/pipeline/alignment.py", line 37, in <module>
    "bwa": NgsTool(bwa.align_pipe, bwa.align_bam, bwa.galaxy_location_file, None),
AttributeError: module 'bcbio.ngsalign.bwa' has no attribute 'align_pipe'
>>>

lbeltrame/bcbio-nextgen,lbeltrame/bcbio-nextgen,lbeltrame/bcbio-nextgen