Switch to conbenchlegacy package

[alistaire47]

As part of https://github.com/conbench/conbench/issues/1439, move from installing conbench as a package to the new, much lighter conbenchlegacy that only includes the bits needed (the CLI and the runner), not the entire webapp.

Cleans up some dep specification along the way, but is mostly just changing imports.

[coveralls]

Pull Request Test Coverage Report for Build 5944302429

• 48 of 48 (100.0%) changed or added relevant lines in 18 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 87.338%

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Totals
Change from base Build 5414429193:	0.0%
Covered Lines:	1552
Relevant Lines:	1777

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💛 - Coveralls