Preserve extras when building local Linux requirements.

[freakboy3742]

As Linux installs need to work with Docker, any reference to a local package is converted into a wheel, and the install requirement converted into a reference to a local wheel install (relative to the bundle).

However, if a package specified as a local source package references extras (e.g., ../path/to/code[extra]), this fails, because the [extra] part isn't part of the file name, so the project can't be found.

This PR modifies the lookup strategy to split off the extras, and perform wheel/path lookups based on the "base" part of the name. The install requirements are then re-written on the fly, appending the extra definition, and passed to the underlying install tooling.

This also modifies the build to use wheels for local builds, since all Python installs will be PEP517 builds now anyway.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct