Allow local paths to be used for package specification.

[panno8M]

Same content as PR #1237. I reworked it because I accidentally committed an extra one.

What this PR will allow to do

• Allow local packages to be specified using ordinary paths
• Deny local package cloning by specifying local path + --withDependencies when nimble develop.

This reduces confusion like #1134 and #1124.

Case Study

Install local package

# before:
cd path/to/pkg && nimble install
nimble install file:///absolute/path/to/pkg

# after:
nimble install /absolute/path/to/pkg
nimble install relative/path/to/pkg
# + before

Develop local package

# before:
nimble develop file:///absolute/path/to/pkg # -> the pkg will be cloned into subdir

# after:
nimble develop path/to/pkg # -> the pkg will be cloned into subdir
nimble develop path/to/pkg --withDependencies # -> the pkg will NOT be cloned. nimble.develop points to original.
# + before

Things to consider

Maybe we should have an option to clone local packages or not.