dependency pinning: luarocks.lock file and --pin flag

This adds support for pinning dependencies in projects and rocks:

* Adds a new flag called `--pin` which creates a `luarocks.lock`
  when building a rock with `luarocks build` or `luarocks make`.
  This lock file contains the exact version numbers of every
  direct or indirect dependency of the rock (in other words,
  it is the transitive closure of the dependencies.)
  For `make`, the `luarocks.lock` file is created in the current
  directory.
  The lock file is also installed as part of the rock in
  its metadata directory alongside its rockspec.
  When using `--pin`, if a lock file already exists, it is
  ignored and overwritten.
* When building a rock with `luarocks make`, if there is a
  `luarocks.lock` file in the current directory, the exact
  versions specified there will be used for resolving dependencies.
* When building a rock with `luarocks build`, if there is a
  `luarocks.lock` file in root of its sources, the exact
  versions specified there will be used for resolving dependencies.
* When installing a `.rock` file with `luarocks install`, if the
  rock contains a `luarocks.lock` file (i.e., if its dependencies
  were pinned with `--pin` when the rock was built), the exact
  versions specified there will be used for resolving dependencies.

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