Implements RFC0013 for generating app source project metadata

[carlo-colombo]

• when fetching the git repository it creates as for Implements RFC0013 for app source project metadata buildpacks/lifecycle#230 a project-metadata.toml file containing information about the git repository
• the layers volume need to be mounted on a different path because if mounted on /layers because the prepare container is built itself using pack and already contains a /layers directory. The specific failure is the launcher failing to read some expected file.
• this change can work independently from Implements RFC0013 for app source project metadata buildpacks/lifecycle#230

Example of generated toml file

[source]
type = "git"

  [source.version]
  commit = "3b5a3...698317"

  [source.metadata]
  repository = "https://github.com/carlo-colombo/java-maven.git"
  revision = "3b5a3d...698317"

[marcingrzejszczak]

Codecov Report

Merging #287 into master will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #287      +/-   ##
==========================================
+ Coverage   67.38%   67.44%   +0.06%     
==========================================
  Files          77       77              
  Lines        3097     3115      +18     
==========================================
+ Hits         2087     2101      +14     
- Misses        762      764       +2     
- Partials      248      250       +2     

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[matthewmcnew]

Thank you!

[thisisnotashwin]

Holding off on the merge until buildpacks/lifecycle#230 gets merged

[carlo-colombo]

@ashwin-venkatesh buildpacks/lifecycle#230 has been merged

[thisisnotashwin]

Thanks for the contribution @carlo-colombo 😄