vendoring: split sync and update make targets

[abn]

The sync target previously regenerated the poetry.lock for vendored
packages, causing larger than necessary change set for patches. This
change introduces the update target to specifically handle lock file
updates, lock target to regenerate the lock file and repurpose the sync
target to only sync as expected.

[sinoroc]

I modified vendors/pyproject.toml as following:

$ git diff vendors/pyproject.toml 
diff --git a/vendors/pyproject.toml b/vendors/pyproject.toml
index 69ccbd0..75d101c 100644
--- a/vendors/pyproject.toml
+++ b/vendors/pyproject.toml
@@ -22,6 +22,6 @@ classifiers = [
 python = "^3.6"
 
 jsonschema = "^3.2.0"
-lark-parser = "^0.9.0"
+lark-parser = { git = "https://github.com/MegaIng/lark.git", branch = "custom_import_sources" }
 packaging = "^20.1"
 tomlkit = ">=0.7.0,<1.0.0"

Then I ran make vendor/sync:

$ make SHELL=bash vendor/sync 
# regenerate vendor.txt file (exported from lockfile)
~/workspace/poetry-core/vendors ~/workspace/poetry-core
# vendor packages
Load configuration... Done!
Clean existing libraries... Done!
Add vendored libraries...  
  Running pip install -t poetry/core/_vendor -r poetry/core/_vendor/vendor.txt --no-compile --no-deps
    ERROR: Invalid requirement: 'Warning: The lock file is not up to date with the latest changes in pyproject.toml. You may be getting outdated dependencies. Run update to update them.' (from line 9 of poetry/core/_vendor/vendor.txt)
  Command exited with non-zero exit code: 1
# strip out *.pyi stubs

Indeed the following has been generated in poetry/core/_vendor/vendor.txt

$ git diff poetry/core/_vendor/vendor.txt 
diff --git a/poetry/core/_vendor/vendor.txt b/poetry/core/_vendor/vendor.txt
index a1c24ba..b6a594b 100644
--- a/poetry/core/_vendor/vendor.txt
+++ b/poetry/core/_vendor/vendor.txt
@@ -1,8 +1,9 @@
 attrs==19.3.0
 jsonschema==3.2.0
-lark-parser==0.9.0
+lark-parser @ git+https://github.com/MegaIng/lark.git@custom_import_sources
 packaging==20.4
 pyparsing==2.4.7
 pyrsistent==0.16.0
 six==1.15.0
 tomlkit==0.7.0
+Warning: The lock file is not up to date with the latest changes in pyproject.toml. You may be getting outdated dependencies. Run update to update them.