Implement `snapcraft update` for parts

[sergiusens]

LP: #1594643

Signed-off-by: Sergio Schvezov sergio.schvezov@ubuntu.com

[josepht]

Is it possible to have this logic in a single place? Same for "parts.yaml" and "headers.yaml".

[sergiusens]

If the duplication is only in the tests I actually prefer that as we would detect the change

[josepht]

Looks good to me.

[sergiusens]

retest this please

[elopio]

integration tests are failing:

ERROR: test_curl_exists (test_parts.PartsTestCase)

test_parts.PartsTestCase.test_curl_exists

testtools.testresult.real._StringException: Traceback (most recent call last):
File "/tmp/adt-run.4uGBaN/build.j9u/real-tree/integration_tests/test_parts.py", line 44, in test_curl_exists
with open(self.parts_yaml) as parts_file:
FileNotFoundError: [Errno 2] No such file or directory: '/home/ubuntu/.local/share/snapcraft/parts.yaml'

FAIL: test_update (test_parts.PartsTestCase)

test_parts.PartsTestCase.test_update

testtools.testresult.real._StringException: Traceback (most recent call last):
File "/tmp/adt-run.4uGBaN/build.j9u/real-tree/integration_tests/test_parts.py", line 37, in test_update
self.assertTrue(os.path.exists(self.parts_yaml))
File "/usr/lib/python3/dist-packages/unittest2/case.py", line 702, in assertTrue
raise self.failureException(msg)
AssertionError: False is not true

[sergiusens]

@elopio yeah, fix it fast, the environment mocking logic was different than in unit tests and was passing locally here as I do have a real cache enabled already