Reduce dependency usage

[ka3de]

Some dependencies currently required by the project are marginally used and could potentially be removed. This acquires more relevance with the upcoming merge with k6 core due to the importance that is placed in this project to minimize the dependencies usage and limit its heterogeneity in order to prevent malfunctions between extensions code.

Some examples for dependencies that could potentially be reevaluated are:

• github.com/hashicorp/go-multierror
• github.com/fatih/color
• github.com/mccutchen/go-httpbin/httpbin: Although in this case this one has 0 transitive dependencies -> Not included in binary as is only required in tests package.
• github.com/oxtoacart/bpool: Only 1 transitive dependency for testify, which is nevertheless used in xk6-browser anyway.

Other dependencies such as:

• golang.org/x/exp
• golang.org/x/net
• golang.org/x/sync -> Only used in tests pkg
• gopkg.in/guregu/null.v3
  Are marginally used but are required as transitive dependencies by other modules.

Additionally this is in interest of reducing the xk6-browser / k6 binary size.

[ka3de]

In regards of github.com/fatih/color usage:

A couple of points of view.
From one side, color library is only used in the logger implementation, and only when the "inner logger" is nil. See here.
But in practice this never happens, as in our implementation, every time that a logger is created, we pass the vu.State().Logger as the inner logger. See references 1, 2(also it's a test), 3, 4.
So we could probably replace this by a simpler implementation from our side.

On the other hand this library is already being used by k6 core, so probably there's no need to remove it as it'll be included in the final binary anyway?

[ankur22]

Re. github.com/fatih/color

color library is only used in the logger implementation, and only when the "inner logger" is nil

Based on how k6 works with dependencies, we should remove it, which would be my preference too.